Sometimes you do more harm by helping than by not.
Windows OneCare is Microsoft's all-in-one personal security suite, encompassing everything from malware removal to virus scanning on your average personal workstation. The latest release has a particularly nasty glitch, though: When scanning your Outlook .pst files, if it happens to come across an infected e-mail it'll move the whole file into quarantine or delete it entirely depending upon how you've got it configured. It doesn't treat a file that is a legitimate part of a Microsoft app any differently from a trojan executable on the hard drive.
Thankfully, there is a workaround for this problem outlined in the article.