Curiouser and curiouser. Thanks to all who responded and special thanks to E-Liam who PM'ed me a tremendously detailed and foolproof list of instructions for getting rid of this thing called CoolWebSearch, which is indeed a nasty little bugga involving a trojan which as I understand it apparently gets into computers still running an early version of Microsoft Java. I imagine from this that XP would be safe, but I may be wrong there. Anyway, all traces of it have now been removed from my computer, for which I am very grateful.
HOWEVER! My original problem still remains.

While trying to compose a message on a web based email, (it happens on Pprune email, Hotmail, and Excite Mail), a post on Pprune and I assume other noticeboards, or even a PM on Pprune, the Enter key still makes the screen inactive; the cursor disappears and I have to click in the frame for it to reappear then use arrow keys to move it to where I want it to go. If I wanted to hit Enter ten times I would have to repeat this process ten times.
Anybody else found this problem?