Windows XP Immediate Logoff after Logon

[Connoisseur]

My home machine running XP suddenly had a long (5 second) beep while I was using it the other day and then restarted. After that, every time I try to log on to my account, it immediately logs me off. I can't get past the logon screen. I tried safe mode into admin and it does the same thing. Anybody know what the issue could be? I'd like to avoid a reformat if possible. It seems to start up fine. My system is as follows:

AMD Athlon XP Mobile Barton (1.7ghz @ 2.3ghz)
1.5GB DDR PC3200
Biostar M7NCD Pro
GeForce 6600GT
SB Audigy 1
4 HD's
DVD-ROM
Thermaltake Silent PurePower 480w

And before anyone suggests it, yes I clocked down to default speed, fsb, and voltages. I also tried a new bios. My CPU temps are about 45C idle when overclocked. It was running fine for at least a couple years on this setting. Any ideas?

 

[Connoisseur]

bump

 

[wpshooter]

Have you added any hardware recently ? I see you have 4 hard drives. Have any of them been added in recently ?

 

[Connoisseur]

I added the 6600GT about a month ago. All the hard drives have been in for at least 3 or 4 months. It was working flawlessly the whole time