Sleep Mode Not Working Correctly

I have a Dell XPS M1530. Since I am a college student, and when I usually leave class, I just close my laptop. let it go to sleep, and put it in my bag. The problem lately has been that when I close my laptop, it does not go into sleep mode, even though it is set for it to do this. It will do this on both battery and AC power. I will put it into sleep overnight so it is in sleep when I put it in my bag in the morning. And once last week I put it in there thinking it was a sleep and then 3 hours later while I was at work, I had not brought it out yet, and opened my bag and the laptop never turned off, it had been running the whole time and was hot, and had 22 minutes of battery left. One odd thing that it does is that it turns off the screen, and logs me off, but it does not turn off. The specs for the laptop are in my signature, and just to note, I have installed 64bit Vista a couple weeks ago, and this problem occurred after I installed it, but not right afterward, it was working for a bit after I installed it, this has just started occurring in the past week. Do you guys think this could be a problem with hardware, or that its a problem with Vista. Thanks for the help, this is a really annoying problem because I don't want to have to shutdown my comp all the time.

 

its probably a graphics driver problem... update the drivers for graphics card FIRST... if it still doesnt work, update other drivers...

do disk cleanup too

 

maybe you have configured sleep mode to do nothing when you close your lid

try to sleep by alt + f4 and then selecting sleep