This worm becomes active on the third day of each month, and it corrupts a bunch of documents. It affects .doc, .xls, .ppt, .pps, .pdf. .psd, .rar, .zip and lots of other file formats.
It's not sure how many computers have been infected but according to security firms it are about 300,000 PCs, worldwide.
To protect yourself you need an anti-virus scanner, and be sure not to open strange e-mail attachments.