Sleep wakes up right away, every time . . .

I have a new XPS 435mt and I've added a second hard drive and better video card. The problem, which was going on before I made those additions, is that when I click on "sleep" and put my keyboard down, my computer wakes within about 90 seconds, every time. Is there something I need to change, or does anybody have a suggestion how to troubleshoot this? I've had the computer for about three months, but have only used it about ten or 15 times because of the hassle of starting up the computer and waiting for all the updates and other actions that happen when a computer has been off for a few days. I'm tired of shutting down vs putting to sleep, and not being able to have scheduled scans and other activity happen overnight, which now I have to wait for them to occur before I shut down. It's just a pain not having the "sleep" function work and since I'm new to Vista I don't know how to fix.  Any help is appriciated.