Do you have backups of your important data?
I suggest download and run the free trial of HDDControl:http://www2.ashampoo.com/webcache/html/1/p..._0065___USD.htm
The program is a time limited but not feature limited trial, and is simple to use. It will show a report of the SMART (Self Monitoring and Reporting Technology) status of the drive. By the sound of it, the drives SMART status will show it is experiencing significant errors, and is likely to fail before too long. If so, it would be best to replace it while it still works, so the current system installation can be cloned across to a new drive.