This is my first time posting on this forum. I built my own computer, using an AMD Sempron 2500, and a Gigabyte motherboard. My problem is that if I start my computer with the motherboard's main jumper off, the computer will start windows, but crash EVERY time I run a program (games, media players, videos). If I leave the main jumper ON everything runs fine, but the bios recognizes the CPU as only 1.0 ghz (it's really 1.8 ghz) can somebody help me?

I believe the 2500+ Sempron runs at 1.75GHz (10.5 x 166), so you're probably setting the FSB jumper to 100MHz & running the CPU at 1.05GHz?

What motherboard model?

Have you checked your CPU temps to rule out an overheating problem?

What speed RAM do you have? Have you tested it? Try Memtest86 v3.2 run from a bootable floppy:

http://www.memtest86.com/#download0

Is your power supply adequate? What's the wattage & how many amps on the +12v rail?

Asus A7N8X-X
1800+ @ 8 x 210MHz
768MB PC3200
Asus A9550 128MB/128-bit
Gamer Edition
WinME/WinXP Pro

The model number is GA-7VM400M (revision 2). The CPU temps are fine. RAM speed is 333mhz (I haven't run the test yet.) My power supply is adequate (I think) it's 10 amps on the + 12v rail.

"My power supply is adequate (I think) it's 10 amps on the + 12v rail"

No, your PSU is NOT adequate...you should have a minimum of 16A on the +12v

Asus A7N8X-X
1800+ @ 8 x 210MHz
768MB PC3200
Asus A9550 128MB/128-bit
Gamer Edition
WinME/WinXP Pro

Would the power supply pose as big a problem as having a bad stick of ram? A friend of mine said this is a symptom of a bad ram stick.

Your system runs OK with your CPU underclocked, correct? A lower speed CPU needs less amperage than a higher speed one.

Like I said, 16A on the +12v rail is the MINIMUM recommendation for a system these days. The PSU you have is fine for older systems because they don't rely on the +12v rail to power the CPU. But it's not good enough for an Athlon or Pentium 4 system because the CPU gets ALL of it's power from the +12v rail. If your CPU was running at the correct speed, it would need about 7-8A alone. A single HDD needs about 2A from the +12v, so you're already at the 10A limit. An optical drive needs at least 1A & cooling fans use about 0.25A each. My guess is your PSU isn't cutting it.

You can easily test the RAM. Just download Memtest86 v3.2, unzip, click on Install.bat & follow the instructions to create a bootable floppy...then just boot off the floppy & let it run for at least one complete test series, it should take at least 30 min. If you get any errors at all, replace the RAM.

http://www.memtest86.com/#download0

Asus A7N8X-X
1800+ @ 8 x 210MHz
768MB PC3200
Asus A9550 128MB/128-bit
Gamer Edition
WinME/WinXP Pro