Am using win xp, when I open start menu on my desktop and then for e.g. programs-accessories- system tools, or any other program comp freezes up,can only access task manager by using ctrl alt del,which shows nothing happening, moving cursor over this window brings up egg timer icon, then have to switch off and start again. Really grateful in advance for any help!

Try booting off the Windows-XP CD, and choose the option to 'repair' a Win-XP installation.

Thanks for replying urmod4u,unfortunately xp came already installed on comp,only got recovery c.d.

That recovery CD may do the job, although it also could just wipe out the HD and restore the HD to factory. Did you try it?
Otherwise, I don't think that there are legal objections in borrowing a friend's WIN-XP CD to repair your installation - you *do* have a licence for your computer after all.