|Are you getting no video at any time, or just no video after Windows starts to load?|
If you DO get video until Windows starts to load....
When you have a hard drive that has had XP installed on it when it was connected to one mboard, then move it and try to boot XP when it is connected to another mboard, if the hardware on the mboard is more than a liittle different from the other mboard, XP will not load all the way - that's perfectly normal. Typically you get video while booting until XP starts to load, you see the first bit of Windows graphics, then a black screen with a blinking cursor top left, and nothing further happens. You can fix that by running a Repair installation of Windows, by booting with the XP CD and choosing the SECOND Repair choice - you personal data you have added to the partition Windows is on will not be deleted.
If you're getting no video at any time.....
You haven't mentioned your power supply.
If the hard drive has Windows on it and you're booting from it, if it's led is not blinking like it normally does while booting the computer (if you normally have to Login it will stop blinking shortly after that point) , the most probable thing is the power supply is failing and because of that the mboard cannot fully boot and you don't get video when that happens.
If the hard drive led IS blinking normally...
E.g. Some recent mboards will not produce video from a card in a PCI-E slot by default because they DO NOT disable the onboard video when a video card is installed. You have to connect a monitor to the onboard video and change settings in the bios Setup.
Most mboard bioses beep once while booting the computer early in the boot when the POST completes successfully - are you hearing that? (A speaker or other sound device must be connected to the mboard, or on some mboards they don't have the pins for that and in that case your amplified speakers must be on and plugged into the correct port - green for a single pair). If you aren't hearing one beep, since you've tried two mboards, it's likely the power supply is dying.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this:
If it is failing, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD!)
You can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements.
There is probably nothing wrong with the original ram.
Trying ram you haven't confirmed to be compatible can cause problems.
Contrary to popular belief, it is extremely rare for ram that was working fine previously to go BAD, unless you have damaged it by something you did when installing or removing it, or unless it was damaged by some event such as a power failure or a power supply failing. Almost always, when you have a ram problem, it's either because the ram has a poor connection, or you have installed ram that is not compatible with your mboard's main chipset, or it's CPU's memory controller if that applies.
Ram that works in another mboard , or any ram you buy or have lying around, may not work properly, or sometimes, not at all - even if it physically fits and is the right overall type (e.g. SDram, DDR, DDR2, etc.; PCxxxx, xxx mhz) for your mboard. In the worst cases of incompatibilty your mboard WILL NOT BOOT all the way with it installed, and the mboard may not even beep - the ram has to be compatible with the mboard's main chipset, or in the case of recent mboards, compatible with the memory controller built into the cpu.
The same applies for testing your ram in another mboard - the ram must be compatible with the other mboard - if it isn't, any results of testing the ram on the other mboard cannot be relied upon to be valid.
If you still have the ram that was installed when the system worked fine, try installing just that ram.
See response 5 in this for some info about ram compatibilty, and some places where you can find out what will work in your mboard for sure:
Correction to that:
Once you know which module ID strings work in your mboard, you can get them from anywhere you like that has ram with those ID strings.
If you have brand name ram, it is usually easy to look up whether it's ID string is in a list of compatible modules found by using your mboard or brand name system model number.
If the ram is generic, that may be difficult or impossible.
Installing incompatible ram can cause no boot, no beep, no video. Your mboard appears to be dead.
It is easy to test for incompatible ram that has caused your mboard to fail to boot.
Make sure you have a speaker or speakers or the equivalent connected to the mboard so you can hear mboard beeps (see your mboard manual if you need to).
Remove the AC power to the case/power supply.
Remove all the ram.
Restore AC power.
Try to boot.
If nothing else is wrong, you will get no video but you will hear a pattern of beeps that indicate no ram is installed, or a ram problem.
E.g. for an Award bios or a bios based on one, that's often a beep of about a half second, silence for a half second, a beep of about a half second, silence for a half second, continuously.