yerom,
Firstly, make sure that you have updated Avast to the latest database. The virus you name W32:sdbot-545 was included into the VPS 0432-1 for detection.
If you have an older database, then Avast will not be able to detect and remove it.
Secondly, once you have removed it......
if it comes back, then you have not eliminated the source that gave it to you in the first place.
As has been mentioned, IRC and File sharing are the most common ways that this trojan is spread.
Do you use Kazaa or any similar utility? What about music sharing?
Make sure that all Windows updates are downloaded and installed. Always reboot after the updates to get all your programs to "settle in" again.
Let me know how things turn out.