If you want teche advice on how to secure you computer. then if your *realy* parinoid, and I mean realy, realy worried aobut this then I would do this.(note: Following points only if your are VERY determaned, they are diffcult to do and can be risky) In no perticlar order.
1. Backup your data and nuke your hard drive with a secure file deleteion program like
dban(Darik's Boot and Nuke) to delte your hard drive(
WARNING: VERY RISKY: Only do this if you have NO OTHER CHOICE and make sure you back up all your data because if you use this program on the dod(Department of Defence) Mode or Guntman Mode you are not getting your data back of this hard drive EVER. Just a little fore warning there.... :sweat )
2. Re-install your Operateing System and put your backed up foles back on
3. Create a new IM account to be safe, don't use an online handle(name) that you usaly use.
4. Use an encryped IM service, You might be able to find an IM program that is purely Peer to Peer(that means that instead of Sending your IM to say Microsoft in the case of MSN who then send it to your GF that your chatting with. It would send it directly to your friend)
5. User TOR, google it, it an anoymity service
6. Consider useing an anoymiseing Live CD like "anonmy.os", again google it.
7. Consider useing Linux if you currently use windows.
8. Encrypt private data with a strong encryption program like PGP or TrueCrypt.
As I said, that was in no pertcualr order. and be very carefull with that DBAN thing. The "Department of Defence" Deletion mode is adiquatly named. It delteds ALL your data on your hard drive, Operateing system like windows included, to a standard that the department of Defence use. After that it's not comeing back.