Those who use MSN Messenger should be careful as there is a worm out there which has infected contacts offering you a .pif file which will infect you.
It seems to have mutated as at first the file was called cute.pif but now it changes the file name randomly. I have had offered:
The Cat And The Fan piccy.pif
Crazy frog gets killed by train!.pif
Mona Lisa Wants Her Smile Back.pif
Me on holiday!.pif
When you ask the contact what they are sending you, it seems to block the message so if you get a contact offering you files through MSN Messenger, best bet is to email them to let them know.