Just read this on CNET. Very serious, although if you keep up with the MS patches you will be OK. These pop-ups are the one you see on Drudge all the time that emulates a Windows Pop-up window and says you have computer problems.
I suggest you install all MS patches ASAP.
Ever try running MS's Windows update? That piece of crap looks for ways not to run on your machine. XP, anyways, and MS's help is of no help at all.
I get better info from sites not affiliated with MS.
..While I suggest a better alternative, due to this CERT advisory.