if your system is freezing up. it can be a lot of different things.
i'm going to assume the computer is somewhat old. which means...you should treat your computer like an older pet.
First off. make sure the computer is clean. dirt, dust, and funk build up around the fans... which are normally on the back, or sides of the computer. you'll either need some canned air. Or some sort of compressed spray. If all else fails. use a vaccume, or hand duster, or you fingers/hands to get the dust/dirt off.
The canned air is best, because it'll blow the dust off the fans, and loosen and remove dirt from inside the machine.
If you feel confident, open the computer case, and blow out the dust from inside the computer. Dust... and dirt. clogging up the computer contribute to heat build up.
Heat... and over heating. Is one of the main things that cause a computer to crash ...or freeze.
A computer needs air flow... over the CPU (chip) Hard drive, and system board... to ensure the temperature stays at a decent/safe level.
Also... while you're cleaning the fans, and computer... make sure the fans work. If they're not. That's a reason heat is building up too.
Now... if the computer is cleaned. And it's not heat that's killing your comp. It's some hardware that's failed... or fails... or is failling while you're trying to use it.
For older computers... the power supply might be the first thing to go. Computers need specific amounts of power to run different parts. Different voltage amounts to run correctly, or whatever...at if the power supply can't meet that amount... the system may freeze or crash.
If the power supply is bad. you're only option is to replace it. by buying a new one.
If it's not the power supply... suspect the Hard Drive. Bad sectors or corrupted information tend to lead to stubborn and problematic Hard drives. You can try and run virus scanners, spyware scanners, the windows Defrag program, disk clean up program etc.
if all else fails... you can completely delete/wipe the hard drive... and reinstall a fresh copy of windows. basically... starting over.
You'd need to back up your data first. So you don't loose anything. But the best way to fix a hard drive... is to reset it to zero... by erasing everything, and starting over.
If not the hard drive... your ram may be shot. you can go online... and download or find ram testing programs. that'll determine if your ram is failed, or failing at certain lvls.
If not the Ram... maybe the CPU ... too much heat... over time can cause the CPU to be utterly frak'd.
for any of that stuff... you'd probably have to just replace it.
So...really.
Clean the machine. get some canned air. $5 or so... it's really the best thing you can do. blow out all the dust, and crap off the fans, and the internal parts.
And then, either really cut down on the junk on your computer (old programs... toolbars, old files, porn, stolen music, old games, redundant programs... multiple chat programs. AOL when you don't use it. Limewire, and several other torrent programs, anything that's "free" that you don't use... delete old clunky anti-virus software for more modern low load software ...etc etc etc) and or completely wipe your HD and reinstall windows.
Your system freezes. either when you do something your computer can't handle, or something on your computer fails. Typically heat is the culprit. or a failing hardware component.
But also...as a side note. If your computer is older... you can't try and play the most current video games, or run office 2007 on a machine built in 05 etc. etc.
hope that helps somewhat.