|" it but accidentally changed the letter on my C drive (it was called "O" when I followed directions below."|
"Disk Management will not let you change the drive Letter of the System or Boot drive which is usually C. because it will prevent the system from booting. "
"You can't by default change C to any other letter the last time I checked."
You can't change the drive letter of the partition Windows loads from. That's usually C, but not necessarily !.
You can change the drive letter of any other drive, EXCEPT for that drive letter for the partition Windows loads from, in Disk Management, to any available drive letter. If the drive letter you want to use is being used, you can change the drive letter of that drive to some other drive letter to free up the drive letter you want to use.
XP's (and 2000's) Setup assigns the drive letter for the partition Windows itself is installed on according to whether not Setup detects that other hard drive partitions that have already been assigned drive letters when Setup is running.
If Setup doesn't detect any other such hard drive partitions, if assigns C to the partition Windows itself is installed on.
If Setup DOES detect any other such hard drive partitions, it assigns the first available drive letter alphabetically to the partition Windows itself is installed on., NOT C !
If O were assigned to the partition Windows itself is installed on, you COULD NOT have changed it !.
You're changing that drive letter has nothing to do with why your computer isn't booting properly !
Your computer is not booting normally for some other reason !
Is the mboard booting fine, you have video before Windows is supposed to load ?
Are the power led and hard drive activity led lighting up "?
Does the hard drive activity led blink like it normally does while booting ?
Does the mboard been once or twice like it normally does while booting ?
Do you hear the CPU fan and hard drive spinning ?
Is the power supply fan spinning and blowing air out of the back of the case ?
Do you have, or can you borrow, an XP CD to boot the computer from ?
If you can there are many things you can try to get your system working properly again, without having to install Windows from scratch !
If you install Windows from scratch, if you want XP to assign C to the partition Windows itself is installed on for sure, if you DO have other hard drive partitions drive letters are already assigned to, there are things you can do that will make that happen for sure.