Try this it helped for me
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Clear Vista Memory Cache:
If your Vista computer is feeling a little sluggish after being up for a while, a thing that sometimes helps is clearly the memory cache. This helps free up unused RAM, and having more available RAM can make your computer faster. For me, I keep a shortcut to this command on my desktop so that whenever I need it, it's just a click away.
To create this shortcut right click your mouse on an empty area of your desktop and select New -> Shortcut. When you are asked for the location of the shortcut, enter the following:
%windir%\system32\rundll32.exe advapi32.dll,ProcessIdleTasks
then click Next. You can then assign a name and an icon to the shortcut. Next time your computer feels slow, click on this icon to free up available memory and hopefully your computer will be faster afterwards.