System freezing shortly after startup.

Recently I have been encountering a problem where; after turning my computer on, allowing programs to load and beginning use of my computer e.g. internet windows(Firefox mostly), the screen essentially freezes.
All icons, windows, keyboard keys etc become inactive except for the mouse which still is able to move the cursor but it has no function. When I move the cursor over the taskbar I get the double ended arrow cursor.
It's not a major deal and doesn't happen all the time but is annoying because I have to do a 're-set' and in the process lose any pages I have open.
Any ideas on this?

I'm using an ASUS P5W DH deluxe m/board, Windows XP, Firefox, Windows Explorer.