That's not a lot of information to go on. Are you running low on either memory or disk space? How much of each is free? If your Mac is running low on RAM, it swaps bits of data to the hard disk to free up memory and keep going. The down side is that hard disks are much slower than memory, and if your disk is getting close to full, it's EXTREMELY slow.
You may also want to check your hard disk to make sure it's okay. Mac OS X usually needs no maintenance, but you can check on and repair your hard disk(s) using a program called Disk Utility (in Applications -> Disk Utility). When you run the program, first thing to do is 'repair disk permissions' which sort of does a basic check of your system and installed applications to make sure everything's in order. It will find and take care of any mistakes or minor issues without needing anything from you. Next, you can check to make sure your hard disk is okay by selecting the drive and clicking 'verify disk'. If it finds any issues, then reboot to your system startup disk, run disk utilities again, and choose 'repair disk'.
That's all I can think of offhand, without additional info. Good luck!