Sorry to hijack your thread with yet another long one, but your experience is not unusual, and many don't know what to do when they accidentally compromise their own security mechanisms.
You took action to protect yourself by downloading a highly recommended and excellent product. Unfortunately, you probably got it from a less than reputable source.
Lesson 1: Download and install at your own risk. Objects on the web are not always what they appear.
Lesson 2: Get your software from a reputable trust worhty source. Use one that scans and screens their software. In my twenty+ years playing with PC's, C-net's sites like download.com have never let me down.
Lesson 3: Scan everything that you download before installing it. Norton can be configured to detect common trojans (not all) and other malware.
Lesson 4: Read the docs that come with the new app BEFORE installing it. Anti-virus products may have to be DISABLED for many installations to complete. They should never have to be UNINSTALLED though.
Lesson 5: If you have XP, make sure that you do a System Restore Point before installing, and make a copy of your registry in case restore points get corrupted or destroyed. This is becoming more and more common in malware.
Lesson 6: Get all of the tools that you need. Use and become familiar with things like HijackThis. It will help a great deal in idenifying offending programs, removing them, and remediating their damage.
Lesson 7: Backup your data files often, and DO NOT leave the backups on a medium that is easily accessible. For instance, if you backup to an external hard disk, don't leave it plugged in and turned on unless you are doing a backup or restore!
Lesson 8: Be prepared to re-install your OS. If you are even remotely unsure if the program has been removed or has installed something else somewhere, blow it away, and start clean.
Lesson 9: When you install again, disconnect from the Internet and follow a checklist. It should outline the following steps:
1) Disconnect network cable from NIC.
2) Install XP from known good CD, removing, recreating and formatting the partition. Don't use quick format.
3) Apply all of the Service Packs that you downloaded to your backup media for this purpose.
4) Apply all of the patches that you downloaded to your backup media for this purpose.
5) Install an Anti-virus package.
6) Apply definition file updates if possible from your backup media. Might not be THE most current, but better than raw.
7) Install a personal firewall.
Install Office or other apps that you use.
9) Update your apps with patches stored on your backup media.
10) Re-connect your network cable.
11) IMMEDIATELY do a Windows and Applications Update. Don't go surfing the net! Be patient.
12) Reboot to be sure that all patches and updates are fully applied and activated. (If previous patches recommeded reboots before continuing, REBOOT! There is usually a good reason!)
13) Update your virus definitions file and firewall.
14) Take the time to download and save all of the new updates to your backup media.
15) Create a new XP install CD that incorporates all of the patches and service packs that you have downloaded so that there is less to do next time.
Lesson 10: There will ALWAYS be a next time. I run multiple firewalls, have tons of security tools and 20 years experience. I re-install my OS and apps every few months regardless of known or suspected compromise, more often if needed. With all of my protective actions, I still occassionally see a security incident. Just last month my defenses picked up something that had exploited a vulnerability in one of my apps that uses SQL that was attempting to copy a password cracking file onto my hard disk. It was caught and removed, but the fact remains that it got through that far. Format and change all passwords on all systems, apps, switches, routers and firewalls.