If you mean that it is taking a long time to carry out commands or you keep getting blue screen problems with a white error message ( might look like: 00000000XXXXEE11 or similar), then you have two possible problems:
Software relatedFirstly Software related:
Slow running is often caused by the hard drive getting too full up. Windows likes to use some of the hard drive for a "swap file" (an area that it uses to store and retrieve information on as it carries out commands.) the bigger the swap file, the better it runs.
Also, over time, windows "throws" files anywhere it can on the hard drive, after a while, these take longer to find and the system slows down.
To cure this, in My Computer / C Drive / right mouse click and defragment the hard drive, then run a check for errors (not surface scan). Delete any cookies or other unneeded files from the hard drive and back up or move to another external drive files you do not use.
Worth at this point considering getting another hard drive to install in the computer / or an external USB hard drive.
After this, run a virus scan, to ensure that there are no viruses on the hard drive. You can download a good scanner fro free from DR WEB (cure it).
After this, if you are having the same problems and are getting the error message described above, check to see if you have installed some software recently or a driver for some hardware that may not agree with your system.
After that, it is possible that some windows files (.vxd possibly) have become corrupted. At this point do an over the top repair install using the windows cd.Hardware problems
These can range from the RAM on the motherboard beginning to fail (remove one module and check with only one installed). Replace if necessary
To a clogged CPU heatsink, blow any accumulation of dust and debris away from the heatsink to stop it overheating.
To (and this is a big possibility), a failing hard drive. You can test this by downloading a programme called HDD tune (free) and running it to see if the hard drive has any bad sectors or is failing.
After that, consider installing more RAM into the computerhttp://www.freedrweb.com/cureit/?lng=enhttp://www.hdtune.com/