CCleaner is probably one of the few Registry cleaners that I'd dare use. Just make sure you back up your registry before you use it (CCleaner will prompt you to do this)

Besides those tips above I'd like to add a few suggestions.

Run a quick or partial virus and spyware scan at least once a week. You should do a full scan at least once a month.

On clearing up disk space you can do the following things:

Remove temporary Internet files

Empty the Recycle Bin.

Delete downloaded program files.

Remove Windows temp files.

Uninstall unused Windows components.

Uninstall programs that you aren't using.

delete old restore points and shadow copies from system restore utility.

Also

Find and run Error Checking utility which will check the integrity of files on your drive.

If you have an old computer your computer's components are obviously going to show some wear. The insides of your hard drive are basically spinning platters which are written to with a little needle. Over time those platters get worn out and it reduces the speed of the drive. They can last for years if taken care of properly, but sometimes it may be necessary to replace your hard drive if it becomes too worn.'

If you still want more speed and you've performed the above steps there are a number of routes you can take:

1. Your system may have been infected with viruses which your virus scanner can't or won't detect (Some viruses are capable of rewriting your virus scanner so that it won't see them). There are a number of bootable virus scanners you can download (preferably from a system that is NOT infected) that you can burn to dvd or cd. You can boot your system off one of these and scan your computer and hopefully fix any problems. You can also run a repair install with your windows cd or repair disk if your computer isn't behaving properly.

2. Sometimes if your system is laden with too many viruses or if you haven't been very diligent with your maintenance you'll experience slow down. If things have gotten really bad sometimes the only thing to do is wipe your hard drive and start with a fresh windows install. ONLY DO THIS AS A LAST RESORT AND BE SURE TO BACK EVERYTHING UP FIRST!

3. Update your system. You'll be surprised how much just adding a few sticks of RAM can increase your system's performance. You can also update your processor (if your using a desktop anyway. If you have a laptop you still may be able too, but you may have to take the whole thing apart), or hard drive (see below). You can also update your motherboard (you probably won't be able to do this with a laptop as most laptop motherboards are designed specifically for that model of laptop) which to a lesser extent can also effect system performance.

4. Replace your hard drive with an SSD (Solid State Drive). They don't have any moving parts like a traditional hard drives so they don't produce any heat (believe me that's a good thing!). They can be pricey, but I can actually boot my computer in about 15 seconds because my boot drive is an SSD.

5. Chuck that old piece of sh** out the window. (Joking, joking :P)