Plz help me solve major Win XP boot problem

system:
chaintech 7NJL6 MB
amd athlon xp cpu running at 1.79ghz
512 mb ram
NVIDIA GeForce FX 5200
excelstor j880 hard drive
a DVDROM and DVDRW
silicon image raid card (driver not currently installed)
lots of anti spyware and virus removal software

xp version: all xp updates installed to the best of my knowledge, but i have had this same problem with or without updates installed, and with any version of XP i have tried
system properties says XP PRO version 2002 SP2

i have had problems with this pc for a long time... a little backstory
my computer was built from parts around 4 years ago, an athlon xp system.
about a year ago it basically died on me. trying to keep my costs to a minimum i replaced one part at a time. eventually i had to replace everything including the motherboard, RAM, and CPU to get the thing working again - basically everything but the hard drive. that seemed to work for a while but the computer would still lock up occasionally while in windows. so, eventually, i bought a new hard drive, installed xp on it, didnt copy any files from my other hard drives onto it in case they were contaminated with a virus. guess what? still having problems.

now the problem is booting. every time i boot with this HD i get a different outcome. the follow all happen in random order and about equally as often:
*computer reboots itself just where XP would begin loading up
*the following error message where xp should begin boot screen (not verbatim): "The following Windows file is missing or corrupt: c:\system dir\ntoskrnl.exe"
*screen stays black indefinitely where XP screen should load (referring to this http://www.quad-resistance.com/tuto/winxp/SNAG-0012.png)
*the above XP screen begins to load but freezes just a split second after it begins to load, the whites of the text faded and not even fully brought into focus
*the xp boot screen seems to be running fine but locks up mid process
*the computer actually boots normally!

so as you can see every time i boot i have about a 6:1 shot of actually booting cleanly. actually 6:1 is probably generous. imagine having to sit at your computer constantly rebooting it for 15-20 minutes or more every time you wanted to use it, each reboot getting a different reaction from your pc.

also my computer will still freeze in xp, but instead of as few as 10 or 20 minutes between lock ups, with my replacement hard drive, once i get the PC booted it will run healthy sometimes for hours and hours.

all your help is appreciated! thanks
Matt