Not to be rude, but... you need to read the forum rules before posting a thread requesting help. It'll give you a LOT of useful advice on searching first to see if it's been already asked (and don't forget the guides), and if you still need to ask, then what all details you need to include so the rest of us stand a snowball's chance (in you-know-where) of being able to answer your question.
Like: What model of Mac is this? Is it a PPC or Intel-based Mac? What version of Mac OS X are you running? How much RAM do you have? What else is going on when this happens? Is this universal, or just in the Finder / specific application / etc... ? How long has it been going on for? What else was going on at and around the time you first noticed the issue? What corrective steps have you already attempted to resolve it?
