These type of spam messages are typically sent by a virus active on the computer from the person who sends them. If you receive them from your contacts, and your contacts receive them from you, then you're both infected.
Get a good anti virus program and do a full scan, make sure your PC is virus free and up to date with all the security fixes from Windows Update.
Some good antivirus programs are:
http://free.avg.com/
http://housecall.trendmicro.com/ (online scanner, doen't remove viruses, but does a good job on telling if something is wrong)
http://www.eset.com/ (not free, but very good)
Even better would be formatting completely (using the discs that came with the pc) and start out fresh. Make sure the first thing you do after that is getting all the Windows updates.
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