I had the same problem but I was lazy and went to the Pro's at mousemovers.com. They installed AVG antivirus for me and removed a shed load of adware and stuff also he used a free peice of software called cleanup by Steven Gould which he shifted loads and loads of unneeded files. Now it runs loads better although they did recommend to get myself a RAM upgrade to sharpen it even more.
Hopefully this is usefull
Simon