If you find that your Windows Vista or Windows 7 is freezing or crashing randomly; the reasons can be many, and the solutions many too ! This will just show you the direction in which you can work on. First see if a System Restore helps make the problem go away.

Next, try the most basic :
Run your updated Anti-Virus and your Anti-Spyware/s, followed by a CCleaner, junk clean-up. Even if the scans dont throw up anything, it might be a good idea to get your HijackThis logfile analysed. See here for more on this.

Next, disable your Aero, and see if its helping.
Else, try disabling the Indexing Service.

Most such Windows freezes are also caused due to Crapware installed on pre-installed OEM Vista machines. So check up which programs you don’t need and uninstall them. Clean up with a Registry Cleaner to clean up residual junk registry entries. PC DeCrapifier is a nifty program which can help you identify Craplets.

Also remove from Start-ups several non-Microsoft items which you think you don’t need them to start up, every time you boot.

Next, check for Error Events : Open Control Panel > Administrative Tools > Event Viewer > Custom Views > Administrative Events. Check if any recent events show a round red exclamation mark error icon. Double click that error and a Event Properties window will pop up. See if the information helps you identify the problem.

For example, if Windows Search Indexer does not work, due to the inability of the Search Service to start, then the system tends to freeze periodically. In such a case it is best to change the Recovery Actions for this service. To do so, open Services > Windows Search > Properties > Recovery Tab. For first failure, choose Restart the service. For second and subsequent failures, choose take No Action option. Apply > OK.

If it still doesn’t help you may have to consider Updating / Flashing your BIOS. Take care while doing it.

Check for Bad or Outdated Drivers.

Also see the Reliability Monitor. It lists the failures and the problems your machine may have faced.

If all this doesn’t help, then I suggest you go through the interesting and fruitful discussion on this rather long thread here at TechNet.

UPDATE: Microsoft has released a hotfix to address such random Windows freezes.

How To Make Windows Faster and Process Tamer may interest you.

Fix Freezes or Crashes with these resources from TheWindowsClub:

Windows 7 freezes | Windows Explorer crashes | Internet Explorer freezing| Google Chrome Browser crashes | Mozilla Firefox Browser freezing | Windows Media Playes freezes | Computer hardware freezes. Need more? Try FixWin, the Windows Doctor!