Vista won't stay asleep

"I've seen this happen before as the result of a mis-behaving bit of hardware or a bad driver. Use the powercfg.exe tool suggested by @Josip to find out which device it is. Then, go into the device manager and find the device listed there. If you check the properties for the device there should be a ""Power"" tab with a check box to ""Allow this device to bring the computer out of sleep."" Make sure there is no check mark in that box.

One other note: with laptops especially there is a risk that you will lose the ability to wake up your computer at all when you do this, so you'll want to do a test first. It's unlikely, but still worth checking out. Test it by making sure you don't have any open programs or unsaved work on the computer and put the computer to sleep, and then make sure you can wake it up normally. After all, you may have just disabled the wake-up feature for the device you expected to use the wake you computer again.

I don't think you'll have any problems, but if you do just hard boot your laptop, go back to the device manager, and look for another device that can bring your computer out of sleep. Something like the keyboard or mouse will likely do it."