Method Of Infection
If it attempts to spread via KaZaA, emule, overnet, irc, ftp, http (and so on) it may pick possible file-names from a long list of names, especially pertaining to cracked software and pornography, in order to try to entice people into running the file. (maybe thats why its in your download folder

)
If it attempts to spread through open shares (like admin$,c$,D$,printer$,ipc$) it may try some password-protected shares using its own list of common user-names and passwords.
some variants are also able to use bruteforce technieks to get your passwords
systems not patched against the DCOM RPC vulnerability or the RPC locator vulnerability.