Wendy, is it a random pop-up, or does it occur each time in relation to a particular event?
If at all possible a screenshot of it happening may help in a diagnosis.
Without seeing it happen, here's what I would do. Ensure I have adequate virus & spyware/adware detection software on my pc, there's a particularly good free spyware/adware program available
here. There is free anti-virus as well as anti-spyware & anti root-kit software available
here. Once you have everything in place then restart your pc, before you get the Windows loading splash screen you want to rapidly press the F8 key, and make the correct choice to enter Windows in Safe-mode without network capability. Once you're all loaded up, then run any anti-virus software and your anti-spyware software. Take care of any problems that present. To do this thoroughly you should really disable System Restore as lot of virus/spyware writers write their malicious software to actually use System Restore to re-infect a clean pc. If you're not comfortable doing any of this, then please seek out someone with a little more advanced knowledge to do it for you so that they can troubleshoot any problems they come across.
It's possible of course that there's a legitimate reason for this occurring, so without further investigation it's difficult to diagnose. The procedure I've outlined above shouldn't cause any harm to your PC either way
