Defragment your Hard Disk Drive !
Like once a week, minimum.

If you can, use Diskeeper and run a boot-time defragment at least once per week.
Other defragmentation options available with Diskeeper, and temporary changes made to your Virtual Memory (Paging Files) settings (immediately before defragmenting) will improve overall performance.
But, not knowing the details of what you have for a system means it is better to not give a lot of detailed 'tips' now...
Would also be good to check your 'normal' Virtual Memory settings and... wow, a really crazy one... the settings in Internet Explorer for the amount of disk space to be 'reserved' for temporary internet files...