If you use Adobe Reader, then I suggest all Windows and Mac users update to version 10. You can get it from their FTP site:
Reader 10 for Windows
Reader 10 for Mac
The smaller of the two files you'll find, is just Reader alone. The larger file has another Adobe freebie (called AIR) bundled in there. I always go with just Reader myself.
What's special about version 10? It has sandboxing. Reader has a long history of security issues. Sandboxing doesn't eliminate them, but it can make them ultimately futile to the bad guys because they can't get anywhere.
I also suggest Windows users check over their system with Secunia's Personal Software Inspector utility, which helps you get security patches for widespread software like Java, Flash Player, QuickTime, VLC, and so forth. Keeping this stuff up-to-date has great real-world value, so it's worth doing every couple weeks or so.