At this point you've installed XP and applied Microsoft updates. You've got your virus scanner installed and working. You have installed your applications and data and applied yet more updates from Microsoft for Office (if you have that). You've run Stinger and everything is clean.
Your machine is as safe as possible. Now if you turn it off and never use it again, it'll stay that way. But you won't, you'll use the dang thing and every time you do, there's some small chance that a new virus will come by and attack your machine and get past everything.
All the procedures and virus checkers will do you no good if you are constantly installing crap (sometimes call 'software') on your machine. One survey concluded that almost half of so-called free software contained viruses. Don't open attachments. Ever. Unless you were told the attachment is coming and you want it. Just because it seems to come from someone you know, doesn't mean your friend really sent it. Viruses often fake the sender addresses.
You are constantly presented with opportunities to install things on your machine. Unless you know exactly what it is and you actually believe it to be safe, do without. You can't afford it. Do you really need all those 'toolbars', those 'joke programs', that 'game' from your neighbor's friend? Every time you install something on your machine, there is a risk you will get infected. Do it as little as you can get away with.
Sure, I install stuff, but only stuff I intentionally seek out and can't do without. Yes, I have Acrobat Reader installed, but not the five other things Adobe wants to fob off on me. Show some restraint. Here's some further reading:
Your virus scanner software will most likely connect to the Net every now and then and get the latest updates to its database. XP will contact Microsoft every now and then and if there are updates to apply. You'll get a 'balloon' in the lower right hand corner that says something about "You have updates to apply". It might even go out and get and apply the updates. If it does not, you should do it yourself.
All this is good, but it may not happen often enough or soon enough. You should make a pro active effort to check your machine. Just like your car needs regular attention, so does the security of your machine. Once a month or even more often, you should repeat parts of this install process. No, you won't need to install XP again, but your regular maintenance should include:
Running any machine connected to the Net these days requires vigilance. Windows has a particularly high burden. If you slack off and start skipping regular maintenance, in time you'll be burned and be right back where you started. Many times you can just start a scan and then let it run while you make supper or sleep or whatever. With a little planning doing regular maintenance will actually require little real time while you sit in front of the computer.
As the ad says, "Just do it".