This can mean one of two things:
1.) You have a very full hard drive and the computer doesn't have enough space to run virtual memory. On a fast computer there is normally anywhere from 80 GB to 200 GB of space on a hard drive that can be used as virtual memory. Don't know what virtual memory is:
http://en.wikipedia.org/wiki/Virtual_memory
To fix this you could upgrade the hard drive to a larger on e of clean out your system.
2.) You have a failing hard drive that my have trouble reading data, therefor causing the system to run very slowly and for it to temporarily lock up.
If you have this problem then you will need to get a new hard drive and a 3.5" SATA hard drive reader so you can copy your files from one hard drive to the next. I would recommend using Carbon Copy Cloner to transfer the files. I would also use this guide to replace the hard drive:
[guide|1766]
If you want any clarification feel free to ask and I will get back to you promptly.
Hope this helps.