Don't open any PDF files anytime soon. Better yet, don't even go to any websites where PDF files may be embedded. According to hacker Petko Petkov, there's a vulnerability in Adobe Acrobat/Reader that lets malware into your Windows box with no prompts of any sort. All you need to do is open a PDF file or open a URL that has a PDF file embedded.
Petkov says that this has been confirmed for Adobe Reader 8.1 on Windows XP, though a comprehensive list of other affected versions has not been compiled or published. It's important to note that this vulnerability has not been confirmed by any third parties or Adobe as of yet. A similar vulnerability for Quicktime on Firefox had been found by Petkov as well, and has been addressed with Firefox's latest release 2.0.0.7.