A friend of mine drew my attention to an article in a paper about a fortnight ago covering this very problem! The problem is caused by trojan horses. They get into your computer via pop-ups and other advertisments.
Firewalls and anti-virus programs cannot detect these, and therefore allow them to pass through and either let them sit resident in your Temp Files or load a file folder on your C:\ drive.
Swoop101 is right about them generating random numbers and letters either for the From Name or domain sites. As there is so many combinations neither Message Rules or Block Sender cannot cope. I must have about 1,000 Blocks but they still get through. I am downloading about 200 to 300 emails a day, and it's the same old sh*t day in and day out.
The article also drew one's attention to a program called Ad-ware 6.0. It's a Swedish program and it is at:
www.lavasoft.de
I downloaded the free version and I must admit it was rather inpressive. It digs right down in all your programs till it finds them. To date it has deleted about 50 files and removed about 30 folders. It was so good I got the registered program which cost about £20!