Try alex g`s suggestion to see if windows upd is disabled, it should be set to automatic.
If that does not work then run sfc /scannow ( note that there is a space between the c and / ).
Start - all programs - accessories - run. type in sfc /scannow and then click on OK.
Doing this might help you repair damaged files, it takes about 10 minutes and it is best to do a restart afterwards;
http://pcsupport.about.com/od/toolsofthetrade/ht/sfc-scannow.htm
Or, try this;
http://support.microsoft.com/mats/windows_update/
If those do not work then you will have to back up your stuff and do a reinstall, maybe, but that is usually as a last resort.
Windows have a lifecycle after which they become unsupported, even if it is a legal version, worth a check;
http://windows.microsoft.com/en-US/windows/products/lifecycle
Regards again, Bob.