I agree with the above. CCleaner also has a registry cleaner.
First of all though, I would have a good look at any programs you can get rid of. Are there any versions of photo editors, games or other software that you no longer use? This will free up some hard disk space; if the disk is very full it can slow things down.
If so, use, add/remove programs to get rid of them. In theory add/remove programs should clear the registry entries for the software but to make sure nothing is left over, use a registry cleaner.
Also have a look at what is loaded at StartUp. When you install software it is not uncommon for it to add itself to StartUp which means it get loaded every time you boot up, even though you may only use it once in while.
There are some essential things that should be loaded - firewall, antiivirus, software but not a lot else.
After that run the antivirus and anti malware progs, then CCleaner.
You could also defrag the hard drive. Windows has a defragger but the are also free ones around.
If none of this helps then more memory is possibly the cheapest way of improving things.
When you said you don't want to reboot the entire system I'm guessing you are talking about reinstalling the OS. If so, it is not that difficult but you do need to be care you do not lose anything important. Possibly the easiest way is to buy another smallish hard drive (do they make small hard drives any more?) and install the OS on the new drive and keep your existing drive for storage.
Dave