if you have anti-virus software [and if not, you should] check if it is doing startup scans and let them run out to completion if it is - and if you haven't recently scanned your entire system for viruses etc do it now as that could be a cause

check [as gary said] if disk is full [> 80% usage] or needs defragmenting

edit: in the main for windows; if you are mac I would still say you should though others may disagree; if linux then you need to head out to a linux forum