I have come across a piece of malware quite frequently lately.  It generates a pop-up about your computer being infected with a virus.  My most frequent encounter was with the Antivirus Soft version.  In a nutshell, the pop-ups are actually the malware and by clicking on anything in the pop-up you are telling it to install thus causing more problems.  If you see a pop-up while browsing the internet about your computer being infected, do not click on it; not to close, not to cancel.  Immediately go to Start -> Shutdown or Restart on your computer and do not return to that website.  If you continue to get the pop-ups after restart, click here for more information about the virus and the location of removal instructions.

As a side note, remember to keep your programs up to date.  Especially Windows and Adobe Acrobat.  Here are a couple of links to update said software:

Microsoft Update

Adobe Acrobat Reader

Also, you may want to consider using alternate Internet and PDF software such as Mozilla Firefox, or my favorite Google Chrome.  A great alternative to Adobe reader is Foxit Reader.