Help of antivirus program
You must be aware of viruses found in computers due to which you may have to face difficulty in dealing with computer program. If you find sudden lockups then check with antivirus program. If you have not installed antivirus program go for antivirus manufacturer’s website for regular updates. There are many lists of web based scan services available and free antivirus programs.

Protect yourself from unwanted spyware, adware, malware etc. (Download link below)
Today we have PC’s infected with malware, spyware etc. kind of viruses not known to user’s. Your computer may behave sluggish, work slowly and cause system crashes, hangs and behave oddly, if you don't protect yourself from all sorts of spyware including spybot, keyloggers, unwanted toolbars, browser hijacking, spyware pop-ups and even all sorts of Trojan, Worms and Viruses.

I've tried every software program on the market to remove the various junk from my PC and registry. And for the first time in ages, I think my PC is finally clean. Here is one powerful and simple software (see video) which is a must for ever increasing threats of spywares.

Please download Spyware removal tool software
and
protect yourself from PC problems / errors.

The above XoftSpySE Software successfully removes Spyware, Adware, Stops Pop-ups, Kills Trojans, Worms, Viruses, Cleans Registry & Program Errors, Removes harmful registry keys and files, Speeds Up your computer, Destroys harmful hijackers, Helps prevent identity theft, Cleans & Optimize your PC

See for free hard drive space.
Now that you have come to know that it is not infected with virus than check out for free hard drive  space  on drive C: SEE TO IT THAT MORE THAN 20% OF FREE SPACE IS AVAILABLE  otherwise congestion may lead to frequent lockups and computer problems

It should not be overheated.
Overheating is caused by fans inside the case, dust getting collected or may be other cooling problem. Check that CPU fan and power supplied fan are working properly and dust is not been collected on them. The temperature can be checked by running mother board monitor. We always try to keep our computer by cleaning its outer portion but inner portion also needs cleaning and to keep it clean (always clean it with plastic vaccum or crevice tools) and never TRY TO OPEN POWER SUPPLY CASE AS IT HAS HIGH VOLTAGE.

Check hard disk.
Suppose that its logical structure is not so good. To find the error, right click on the disk C: icon in “MY COMPUTER”, then choose the “TOOLS” tab, next thing check all check boxes in the “CHECK DISK OPTION” and press the “CHECKNOW” button. Schedule the check next time you restart your pc-select “YES” option and restart  your pc. The check will be done automatically after startup but even if hard disk fails it means replacement.