This warning from Windows Secrets seems pretty important:

Disable QuickTime until Apple releases a patch A problem was recently discovered in Apple QuickTime that could let Web sites silently install malware on your computers. Malicious banner ads with images that infected visitors' PCs have previously run on such well-known sites as Monster.com, NHL.com, and MLB.com, according to Windows IT Pro articles published on Aug. 23 and Nov. 15 (free registration required). This means that even a "trusted site" could possibly expose your system to harm. Because working exploits are already circulating, and the risk is so severe, I'm recommending that you completely uninstall Quicktime until Apple releases a fixed version. Individual users can do this using the Add/Remove Programs control panel.