|Been happening since you got it - or has it just started? If the latter, any hardware/software changes? Have you got internet security (anti-virus etc)? Might be worth a full scan.|
Try looking in event log (start typing ev in the search box, and it will appear above) for clues?
Is this internet explorer? Ie7 or 8?
If you haven't, try installing another browser - firefox, chrome, opera, safari and see if they're any better (when installing don't import anything from existing browser).