A corrupt registry can definitely slow down your system and also cause it to take forever to boot. It isn't the only thing that can do this but it is a common problem. If you have had the system since 2003 and have never cleaned the registry it should be done.
There is one registry cleaner that I like and I received a promotional version of but it is now $30. It is Uniblue Registry Booster. www.uniblue.com
Will that be the cure to your problems? There is about a 20% chance it will cure all your problems but it will help.
There are a myriad of other things that will make your system run slower. The most common these days are "anti-virus" systems that scan everything coming in or going out of your computer. I stopped using McAffee because it basically became "bloatware" and SIGNIFICANTLY slowed down my system. It also drove me crazy trying to pinpoint the problem. It all changed after an automatic update to the software.
Also: Don't run multiple browser sessions or applications in the background of your system. Do your really need to know the current temperature, breaking news and second by second updates of your stocks? Get rid of these and put these sites in your "Favorites" area and call them up when you want to check them.
Also: Get rid of AOL or other services if you use them. AOL especially will bring your system to its knees.
There are other things to optimize your system but they should be done by someone who is familiar with cleaning hard drives, suspending Services and removing programs. Take it to a GOOD shop to have this done. I am not a fan of the Geek Squad but some people say good things about them.
If you will pay over $200 to have your system optimized then I say just buy a new one. You can buy a good quality system for under $700.