I don't see any reason to uninstall Norton AV - apart, perhaps, from the standpoint of its large impact on system resources, which, if yours is a late-model machine with a relatively fast processor and at least 256MB of RAM (though more is better, of course), should be tolerable. Norton AV consistently scores very highly on independent threat tests, right up at the very top, and is hard to beat for effective protection. Your call, though.
AVG Free is pretty good for a free antivirus. It can be downloaded directly from the publisher:
Grisoft AVG Free
Also available for free to individual home users, and reasonably well regarded, are:
AntiVir Personal Edition Antivirus only
Avast! Home Edition Antivirus only
CA eTrust EZ Armor LE security suite (Firewall/Antivirus suite, 1 year free trial version w/full function for Windows users)
Ad-Aware SE Personal is well regarded free privacy tool which also may be downloaded directly from the publisher:
LavaSoft Ad-Aware SE Personal
Honestly, I'm not a big fan of "registry cleaner/tweaker" apps, and I'm particularly unfond of the "Free Scan" and "Free Trial" come-ons which invariably find problems they'll gladly "fix" for you just as soon as you pay for the full version. Just about anything any of them will do for you once you've ponied up your money can be done by truly free software.
Free for non-commercial use,
CCleaner includes a fairly effective registry maintenence tool (which it labels "Issues") along with a superb general cleanup tool. CCleaner is regularly updated, has a good built-in user help file, and offers a responsive help forum as well as support-via-email.
Free, reliable, highly regarded, and feature-laden, long-time antispyware favorite
Spybot S&D, with its "System Internals" tool, effectively handles many registry issues, and is an excellent companion to Ad-Aware SE.
Free, but no longer published or supported by Microsoft,
Microsoft Regclean (Note: This link is to RegClean 4.1a, Build 7364.1, the January '02 final Microsoft release, hosted by licensed mirror
MajorGeeks) deals fairly effectively with many registry issues commonly found in theHKEY_CLASSES_ROOT section of the registry, though that's all it does; it looks at no other section of the registry. Read
THIS before using.
A well respected, reasonably effective, more broad-ranging free registry maintenence tool is Lexun's
Lexun FreeWare RegScrub XP (The publisher appears to have abandoned this program; it too is linked here via MajorGeeks)