When did this start?
Replace any hardware? Added any hardware?
See if you can install/use these applications:
Download, install, run, update and perform a full scan (separately)
with the following two applications (freeware versions are the ones
to use for this):
SuperAntiSpyware
http://www.superantispyware.com/
MalwareBytes
http://www.malwarebytes.com/
After performing a full scan with one and then the other and
removing whatever they both find completely, you may uninstall
these products,
if you wish.
Download and run the MSRT manually:
http://www.microsoft.com/security/malwareremove/default.mspx
Reboot.
Also - no matter the results (although you should come back and
report those too) - come back with more details on the system and
the problem. For example - the exact version of your OS and the
flavor (Home, Professional, etc) would be useful to those trying to
assist you.
Operating system and service pack level is a nice start - if you
wish to be very accurate - there are two things you should provide:
Start button --> RUN
(no "RUN"? Press the "Windows Key" + R on your keyboard)
--> type in:
winver
--> Click OK.
The picture at the top of the window that opens will give you the
general (Operating System name and flavor) while the line starting
with the word "version" will give you the rest of the story.
Post _both_ in response to this message verbatim. ;-)