Everyone uses different software utilities. Get a hundred people and you will have a hundred combinations of AV, firewalls, back-up software etc. So I thought I'd make a list of what I use, and what I find most useful:
AV/Firewall
Currently I'm using ZoneAlarm Security Suite. The firewall is well known and as secure as it gets. The AV is no slouch either and has some good reviews. The anti-spyware is a little flaky, so...
Anti-spyware
I use Ad-Aware and Spybot. I find the two together are complementary and catch anything the ZA anti-spyware misses.
Disk Maintenance
XP's built in defragger isn't all that useful, so I have mst Defrag. It's an install and forget piece of software which goes about its business un-noticed. My files are always 100% defragged without me ever having to lift a finger. For partitioning I have Partition Magic 8. It just does what I need. I also have a freebie program called Unlocker Assistant - ever had a file you can't delete because some program has its paws on it? This will sort that out easy with a couple of clicks.
Anti-spam
ZA has built in anti-spam, but I'd rather not let it get that far. I have used Mailwasher Pro for years and wouldn't be without it. It lets me delete anything I don't want before it leaves the mail server. You can also bounce anyone who is being a pest... The added bonus for stopping spam at the server is you are less likely to receive an e-mail containing a virus. It's no guarantee of course, but I've yet to have one get through.
Back-up
There are three words to protect your pc, back-up, back-up and back-up. To go with my 1Tb of drives, I use Drive Image 7 to back-up my Windows partition daily. I also use it for other ad-hoc back-ups, such as my games partition, or before and after installing new software. I have My Documents on a separate partition. This is backed-up regularly to two other drives using SyncBack; I've not lost a single file since starting to use this. For e-mail (pst files) and office documents I have KeepSafe. Not only does it work as an effective back-up tool, it also keeps revisions. If you make a mistake on say, an excel spreadsheet, you can just reload an earlier version through KeepSafe - what a program!
That's just a few of the programs I have, but they are the ones I would not like to be without.