This is just general advice, not knowing your setup.
Computers often slow down and the hard disk thrashes when trying to write to disk. This can often be caused by:
a. a fairly full HDD
b. too small a swap file
c. not enough memory (RAM)
d. too many progs running in the background (look at your system tray for an idea)
e. anti-virus software (YES!)
f. inefficent settings in startup files
g. multi-tasking TOO much
There are many more.
If you supply as much info as you can about your machine - HDD, RAM, OS, model/speed, percentage of HDD used, contents of startup folder, typical use of machine etc., I'll give you some specific ideas to try - or e-mail me.
Be safe.
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PistonBroke