@ garyhope
First of all, welcome to TS garyhope, an excellent source for computer help, information and participation in intellectual discussions. For your own benefit, to enable optimum assistance, you are advised to enter your system specs in your CP, as asked by the board.
Now, your question hits the nail on the head, or rather a whole bed of nails simultaneously. Moreover, passing from nails to pins, you have pinpointed many major computer braking systems. The theoretical solution is simple, namely tackle each and every one of the culprits. Of course, this is easier said than done, but this is not the right section for giving full instructions on the multiplicity of tasks involved. To that extent, you can find much material in the tuts section - Guides and Tutorials – that’s what it’s for.
However, as a brief and easy starter pack supplementing mike’s (a past tutor of mine) expert advice, here are some things you can do.
If you’re using Vista, the biggest resources hog I know of, go over to Windows 7.
First of all, have a critical look at all the apps you have installed and uninstall all the ones you are not using. You not only improve performance and gain space, but also reduce the target area of malware aimed at you.
Secondly, 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, see relevant thread in this section. If your antivirus is holding up your system excessively, a frequently occurrence, replace it by a lighter one, such as the excellent and free Avira.
Thirdly, stop all apps starting on their own unnecessarily. The best tool I know of doing this is Autoruns by Microsoft SysInternals. Download from MS or e.g. here:
http://www.filehippo.com/download_autoruns/
Leave auto-starting system services alone for now, the expected gains are small and the risks of stopping them far greater.
Fourth, do the much neglected disk defragmentation, but don’t expect spectacular gains, since modern NTFS systems don’t get anything like as fragmented as older Fat 32 ones.
Lastly, or rather firstly, do a backup of the whole system partition, not just the registry, before you start.
OK, I have to stop now, or I’ll be driving the tuts section out of business and some readers out of patience.
If you are scared and/or lazy and want to buy your way out, get more memory. It’s dirty cheap and damn effective.