Here's a website on how to remove subseven.

An AV software will not catch all infections (especially norton ). Check a few posts back and Msmittens mentioned "thecleaner". Thats a very effective "TROJAN" remover. Now, if the person installed a trojan/backdoor, then your AV probably will not catch it. I ran an AV scan on my computer and it came out clean, but when I ran thecleaner, it caught 5 different trojans infecting over 20 files on my computer.

As for sharing, Def. disable it, unless you MUST share a few files. Windows XP by default has a folder shared (My shared Folder or Shared Documents). Inside that folder you'll find 2 subfolders ( Shared Pictures and Shared Music). Most people dont share anything in there.

Good luck bud.