My dad's got this older HP machine that's pretty much gone on the fritz the past couple of days. He finally got a new one, but I'd like to TS this old one and get it operational again. Per my dad's account of the situation, the machine boots into the the OS (Windows XP Home, not sure what service packs are on there), and then reboots. This keeps happening until a forced shutdown is performed.

Any ideas on what the problem could be here?

can it boot into safe mode?

I had this problem and it only got worse until nothing eventually was possible but a complete reisntall. Save your work off the HDD quick and then proceed. I know I lost mine - whole 20GB of it!

I never figured out what it was but that it must have been a conflicting driver initially. Even in SafeMode mine kept rebooting - and soon the blue screens appeared... and sooner later the NTFS.sys errors while my HDD was/is in good condition.

Check your RAM if all else fails. Yet there's many procedures beforehand to try.