Most likely causes are bad hard drive, and bad memory module... unfortunately, I do not know an easy way to test either when the system doesn't boot. You have to put the hard drive in another laptop to test. Same with the memory modules.

windows boots to black screen with mouse

Hi,
This problem has taken me some days to solve. Actions tried were:
Registry restore
Safe Mode
Windows repair install

None of these had any effect.
I did a new install to a different folder which worked. Then using WinPE, with a lot of patience and renaming I set about replacing all the standard system files in the following directories. (from the new xp Installation)
Windows, \windows\system32, \windows\system, \windows\system32\dllcache and \system32\drivers.
None of these cured the problem. Finally in desperation I replaced a folder called \windows\winsxs and I was amazed to see that this worked.
There are several folders within WINSXS but time being against me I havent located the exact file that caused the problem. I simply note this as a cure but would recommend that the simple things , system restore, safe mode, windows repair are tried first.
Incidentally if you have WinPE or UBCD i would advise copying the registry fiiles before repair install is started then you can return to a point prior to the attemt should it fail.
I hope my solution helps someone,Good luck