First, make sure you have adequate backups. If your hard disk is failing this could be a sign. It can be replaced if necessary, but a backup strategy is recommended whether or not you are experiencing problems.
Read: OS X Mountain Lion: If your Mac runs slowly. It applies to Lion as well. The article is very cursory, but be sure to read through to the end. Unfortunately the link there is broken, so use this one: Disk Utility. To do this you must start your Mac from the Recovery System or Internet Recovery.
Rather than ask a dozen questions for more details about your system consider the following. You must be running either Lion or Mountain Lion. Including your version of OS X in your system description would help.
Apple Support Communities contributor etresoft wrote a very useful app to quickly gather certain system information that may help point to a cause of this problem. Go to his website, download and run EtreCheck:
Etrecheck will take a moment to run as it collects its data.
Copy and paste its output in a reply. Do not be concerned about anything that says "Problem" or "failed".
EtreCheck was designed to remove any personal information (such as your computer's name and serial numbers) but if you see anything that looks like an email address or any other personal information that should not be divulged to others, please delete or obscure that information when you post the reply.
When you are finished with EtreCheck, quit the program. It occupies very little space, and you can keep it or drag it to the Trash as you wish.