Hello all. Just to update you - the problem has continued, and in fact got worse. It's now taking many attempts to start booting up, and when it does finally start booting it often just gets to the point when the Windows screen appears and then fails (as before, screen goes blank, HD light on). Even when the system is finally up and running the same thing sometimes happens when I start a program (almost everytime with Firefox, but not at all with IE strangely enough).
I tried the previous suggestion of checking all connections, boards, etc and it seems to have made no difference. If anyone has any further ideas I'd be really grateful - at the moment I'm quite worried by all this. Thanks.