Are you saying that now you cannot get into XP?
Last good configuration from Safe mode options does not work?
Being a custom computer I assume that you have the XP CD.
Boot it & try FIXBOOT as seen here:
http://www.kellys-korner-xp.com/win_xp_rec.htm
Also if you know the manufacturer of your hard drive you can find their diagnostic here:
http://www.tacktech.com/display.cfm?ttid=287
This will be a bootable floppy that you can create on another computer to run on the problem PC.
If you ever get blue screen errors please note the message in detail. It will generally tell you what is wrong.