There are many measures you can take to improve computer performance, some are not very effective, while the list depends on the advisor. But here are a few simple ones that should have a significant impact.
First of all, Vista isn’t an efficient system, consider going over to Windows 7 if practicable.
Now, go to your Control Panel > Add or Remove Programs and have a critical look at all the apps you have installed. Uninstall all those you are not using. You not only improve performance and gain space, but also reduce the target area of malware aimed at you.
Next, replace resource hogs with lighter apps. Typical example is the heavyweight Adobe Reader, which should be replaced by the featherweight Foxit Reader or something similar. If your antivirus is holding up your system excessively, a frequent occurrence, replace it by a lighter one, such as the excellent and free Avira or Windows Security Essentials.
Finally, stop all apps starting on their own unnecessarily. The best tool for doing this is
Autoruns by Microsoft SysInternals. But be careful not to stop anything useful such as antivirus, firewall and system services. If you’re in doubt about something, better leave it.
Of course, occasional disk defragmentation is another good measure, but personally I do not recommend Registry Cleaners, because any gains are small and overweighed by risks of damaging the system.
If you are scared and/or lazy and want to buy your way out, get more memory. It’s very cheap and highly effective.