yep, if you've got £4000 to spend on computers, what's another £40 for a year's subscription to anti-virus software to make sure the computers don't die on you?
pretty good value! Also try to keep up to date with the latest software security fixes, and make sure you have a firewall installed and turned on.

After that you just need to make sure you use your common sense. Eg, if you're looking for software, it's not likely to be less than a few MB in size, and avoid dowloading .EXE files. Humans are often the weakest link in security.

There's also a nice little plugin by mcafee called Site Advisor for internet browsers (I hate to preach, but use Firefox..) that can help you find out about the safty level of an internet site too if you ever try to use them for things you may not be able to find..