Colin,
Before you go and possibly spend a lot of hard earned loot on a new machine, unless you really wish to upgrade, have a think about the problem. If you have the same virus I mentioned in my Post, then get rid of it as quickly as possible as it will hijack your keystrokes for passwords etc. As mentioned by others, there are some free programs that will remove it, although not Macafee or Norton, however I purchased PC Doctor which did the trick.
The other thing is if you feel your machine has been 'invaded' then do not copy any of your programs to the new machine or you will just transfer the viruses. Re load using the original CD's if possible.
One thing you could do is have a look at all the software on your machine and get rid of anything you do not use, or even only rarely use. Be brutal and that will get rid of a lot of stuff that slows your start up. There are also tools that will look at your Registry and remove a lot of unused or even 'naughty' stuff that has managed to get on there. One program that seems quite good is
http://www.glarysoft.com/
However, I am not a computer expert by any means, so if the machine you have is still good enough, then possibly a visit to a computer geek to sort thing out,might be a lot cheaper than a new one. He may also upgrade the various bits of your machine aswell. but again, be very careful what you transfer.
Good luck