a) Clear out all temporay files and folders -- use Crap Cleaner click here
b) Scan for malware spyware and viruses --Free Anti Spyware :-
Superantispyware click here
Malware removal tool click here
Free Antivirus software
Avast4 click here
AVG antivirus click here
c) Clean the registry -- Use the tool in Crap Cleaner its very safe and also allows you to back up the registry first.
d) Pagefile (Virtual Memory) -- Rght click MY Computer - select propeties - Advanced tab - Performance - advanced tab - Virtual memory click change, you can put the page file on a differnt drive (if you have one), click custom size and set Initial size to one and a half times the amount of memory you have fitted i.e. 512MB memory = set to 768MB, set maximum to double your memory amount i.e. 512MB memory = 1024MB click ok.
If your hard drive is full and there is not enough room for the pagefile this can slow down, freeze or even cause the PC to crash (restart).