Computer Won't go to Standby

[Doomsayer]

I had no idea where to put this, so sorry if it's in a bad category.   But bassically,  I used to put my computer on standby every night because it's pretty loud (cheap fans), and suddenly I go to the "turn off computer" menu where you have choices to Standby, Restart, or Shutdown,  and hit standby like I always have,  and This time I get the whole windows preparing to standby thing, but When it would normally go dark,  it just all comes back and it's like I never hit anything. 

Windows XP SP 2

I haven't made any changes to hardware,  and Have run Virus scans in this order
- Ccleaner
- Ad-Aware
- Spybot S&D
- AVG free
all are updated.

thanks for the help guys

[soybean]

So, it seems like something is waking it up as soon as it goes into Standby, right?  And, that's probably what is happening.  The key is to figure what's waking it up. 

I suggest opening Task Manager and take a close look at what's running.  Try killing some processes before going into Standby and see whether you can find what's causing it.

I think you could possible have some malware that's not detected by any of the maintenance tools you listed.  You may need to post a HijackThis log if you can't identify the culprit by using Task Manager.