If you could give us a shed load more detail we maybe able to resolve your issue.
Information that would be helpful would be things like:
- Operating system
- Installed apps ie Anti virus, anti spyware
- Internet connection
- What you are doing and where you are when this happens
- Any trouble shooting you have done
- etc
For starters some simple things to try;
Have you made sure all your drivers and patches are up to date ?
Worth trying are some other anti spyware/virus apps and some system maintenance.
(I'm going to assume that you are using XP even tho' you didn't mention.
Most of these steps will work with any Operating System)
Here's some steps for you to take.
1/ Remove temp files and other rubbish form your system, either with inbuilt Disc Clean up or
CCleaner.
(Disc clean up; open my computer > right click your C: drive > properties > Disc clean up button on general tab. Let it run and select everything)
2/ To make sure everything is running fine also run both anti virus and anti spyware apps (make sure that they are updated first

) (
AVG is good and free AV)
(A couple of good removal tools are
Spybot and
Adaware)
Also helps if the scanning is done in Safe mode as well as normal mode.
ALso an on line virus and spyware scanner is
Trend Housecall
The use of a number of different scanners is a must as they check for infections in different ways.