XP suddenly very slow to load

[infojunky50]

In May I built a new computer- Specs as follows:
QX6700 Quad Core cpu, EVGA 650i mobo, 8800 GT videocard
2GB 6400 DDR2 RAM, Seagate 250 GB SATA hdd
(2) DVD RW decks
Scanned with AdAware, Sbybot S&D, Windows Defender, AVG
XP pro

It ran beautifully until a couple weeks ago. It powered up ok but stopped at the black screen that lists some specs, "hit Delete to enter BIOS, ESC to enter boot menu" etc. I thought it may be the harddrive so I bought a new one, installed XP again and it was fine for a few days then did the same thing. I tried plugging it in a different SATA port on the mobo and it was fine for awhile then repeated the same behaviour. I replaced the mobo battery too because I noticed if it was unplugged/moved it reverted to the original BIOS date/time. It seems fine for awhile then repeats the behaviour. I formatted the original hdd and it's just blank but installed. (I saved a picture in a folder on it to see if it works) All drives are listed properly in MyComputer, no yellow ? or ! marks in Device Manager.
It's in an air conditioned room at approx. 73F and doesn't appear to have overheating problems, lots of free space around it and fans all running fine. NOw here's the kicker:
When it refused to boot up, I plugged in my standby (new refurb) emachine and it did exactly the same thing, wouldn't boot for about 20 minutes. It always started very fast. Could this be a virus of some kind? I was ready to blame the mobo in the new one but when the emachine did the same thing I was really freaked out. They'll sit at that black screen for up to half an hour then they MAY all of a sudden load Windows and work normally. ANYone got any ideas what the heck is going on?