Its called a Blue screen. :hehe:
It happens when you have you have numbered amount of high risk cashe which could be a threat to your computer later on. Those cashes can pass a worm or set a trojan in to your windows root directory called System32. And then, well…a worm would be placed in your temp folder, multiplies as soon as it gets in your computer and allows other high risk cashes such as spyware to enter so your computer would run really slow. Trojan would just set a timer as soon as it gets in System32 file and will delete it at certain time and date (you cant see these files even in temp file)
To avoid this from happening? Do the following.
Clean your Computer’s hard drive.
I use CCleaner to do this.
and run a virus scan. AVG is one of the freeware you can download from the net.
I also use defraggler so I would know that the memory which looks empty to you isnt really empty. 
Then I use Cleaner again and delete the unecessary files from my computer and run a virus scan again. Its kinda long process I usually do on the weekend ever since I got the warning from rogers. 
A blue screen would also show up if you are using too many programs at once. It will just stop working and dumps your physical memory (virtual memory) and restore your computer from the last upgrade you’ve made.
EDIT: It happens alot ever since vista home edition >.<