anti-virus software

I would recomand to try (for the old troyans) a virus scanner that is accompanied by a cd using linux to start the system and do an offline scan. The big advantage is that it will catch also rootkits and that the cleaning is less risky for the operating system. This applies especially for rootkits, trying to get rid of them while windows is running is often damaging badly the OS. F-secure (on payment) is a good choice.

For antivirus scanner it is a bit like with the rest of it. What costs little is worth little. This depends however on what you do with your computer and on your user profile.

Good virus scanners (not the firewall solutions that are genuine crap IMHO) are: F-secure, Gdata, Kasperskij. Between the free ones I have seen that Avast is not bad (but surely not the best).

On spyware solutions: be aware not to install spyware to get rid of it. There are a lot of "tools" around that claim to clean your system from rootkits. This is nearly never true and often they try to install spyware right away in your pc. Between the realiable solutions I would suggest Spybot.

Besides the obvious rules that apply (not to open attachments of people you do not know, not to install all this "free" crapware with spyware integrated, I would point out not to use in any case two virus scanners on the same machine. In the worst case scenario they would annihilate their efficiency exposing you to noteworthy threads.

Now if you are really on computing and if you did install windows from the scratch and did not have problems with it you may consider the third way.
You install once again in a limited partition size windows from the scratch.

You define a second partition for your (previously backupped!) data and then you use the rest of the disc to install Linux.
As for the distribution: there are many, a question of taste. Personally I prefere Opensuse with the KDE desktop (the newest version comes out in a month and for office work you would not notice the difference to linux (it is just more stable, does not have viruses and has more software and costs nothing). Another very loved distribution is Ubuntu with a Gnome desktop (just another interface, called also Kubuntu with KDE).

So for games and your perfered programmes not available maybe for windows you can boot the system in Windows (with a free antivirus).

And for the internet, especially for email (virus free) you could use the linux partition. You gather the capacity to use a linux system (looks good even on your CV) and for sure you will notice that once the troyans are not there anymore and the system (like linux) has been designed from the scratch for the internet and network, all will be ...MUCH FASTER.

If you decide for this option let me know, I can give you more tips and tricks prior to putting it into action.
In any case have a lot of fun!