Please don't post in html.
Anyway -- it's the sobig.f virus. According to
http://securityresponse.symantec.com/avcenter/venc/data/w32.sobig.f@mm.html
it scans a variety of file types on the infected machine to find email
addresses to abuse.
It's not always a .pif file; sometimes, it's a .scr file.
--Steve Bellovin, http://www.research.att.com/~smb