I should think the most likely reason is that unfortunately its one of those few trusted people that has/had some sort of worm on their pc. :/
Once resident on a PC the worm specifically searches the infected drive looking for contact details. Depending on the worm variant it will usually look for either email addresses are most common as they are easily identified by the @. msn contacts though are another common target, as are mail lists address book etc on the host computer.
Once the virus has compiled its list of contacts from the infected PC, it then targets those other addresses either to continue the replication process (hoping it will reach further unprotected PC's and continue to spread).. or it could be just plain spam.
The klez variant of worms are very adept at doing this sort of thing (I only mention klez because its one of the common worms that I investigated whilst doing my dissertation, but there are many similar types which can also do this) klez became infamous due to its mail engine and ability to spam and replicate. They're not so much a nasty horrible wreck the PC virus.. just damn annoying.
Depending on the type of virus and the spoof .. sometimes if you look closely at the full email header you can sometimes suss some clues as to where its really come from.
Many years ago I had something similar, and I identied it to my uncles PC by looking at the header message source which in that particular instance quoted his PC name. Depends how clever the worm is though at hiding its tracks I guess.
