Ccleaner is free and it gets 10 out of 10 from me, but might not speed up your PC. Maybe more RAM?

http://www.ccleaner.com/

Another very good program is Tuneup, but it’s not free, but for the money it’s excellent.

It has a Turbo Mode, which disables unneeded processes and concentrate all the PC processing power on your active program!

http://www.tune-up.com/products/tuneup-utilities/

Everything needed to speed up a PC is actually available in windows, and/or are good practices:

- msconfig (just type "msconfig" in the run... box) has got a "Startup" tab which lists all programs running at startup. Remove every program that you don't want.

- defragmentation tool (right click your system drive and choose "properties>Tools>Defragment now..." This really speeds up the system.

- Disable/remove file indexing (windows live search, Google desktop)

- The system partition (usually C:) should not go over 75% storage use.

If your PC is still slow, format and reinstall the system. If it's still slow on a fresh install, buy another PC or look into upgrading hardware:

- RAM if you have less than 1GB (2GB if you run Vista)

- Hard Disk (especially in laptops, choose a PMR 5400 or 7200 rpm)

- CPU and/or Motherboard. 2Ghz processor is usually enough for most uses.

Good to know, and very precise, thank you.

I use Tune-up Utilities in combination with SpyBot - S&D. Tune-up really helps speed up the system and continuously works in the background to improve performance and fix problems. SpyBot (which is freeware) actively protects against spyware (and is really effective at it).

You could also try Fix-It Utilities 10 or SystemSuite Pro 10. Both of them offer Antivirus & Antispyware as well as other tools to improve the performance of your computer.

To improve the speed the Windows, you must disable services what you didn't use anymore. (http://webtoolsandtips.com/make-windows-run-faster/increase-windows-speed-by-disabling-unused-windows-services/)

Or read this good article: http://webtoolsandtips.com/featured/make-windows-run-faster/

You can try also this good product: PowerSuite 2010 (http://www.uniblue.com/software/powersuite/)

Try SystemSuite from Avanquest - you can download it from the Avanquest website and it's got a 60 day money-back guarantee, so you can always get your money refunded if it doesn't help.

SystemSuite

Pierre Vebber has given you some suggestions there which I would highly reccomend. Theres nothing worse than powering up your machine, you start to use a program but its so slow as other applications are still loading in the background. Make sure you dont have everything starting up on start up! Has helped me in the past!

What about another operating system? Linux anyone? ;-)

Forget about it, Linux desktop is a bad idea ;)

@Pierre : 2nd option is more memory - 2Gb minimum for XP or Vista - Windows 7, perhaps more but it does seem quicker anyway

@Paul: agree on Vista, but not necessarily on XP or 7. 1GB should be enough, depending on the software you use. Of course, for big apps such as media editing (Photoshop, Cubase, Premiere) or games, 2 GB is always a good thing. Nowadays they sell systems with 4GB, which I think is totally overkill: it's a marketing honeypot, as RAM just costs a few bucks today.

@Pierre : We always find ways to use the memory - we run more applications together, we try out big applications, every other program we install loads its own little gadget on startup.

I remember the company I worked for stated selling PCs with 32Mb hard drives in place of the 20Mb ones we'd been supplying. We wondered what people would do with all the space. A few years later a customer came back with a PC because the 32Mb drive was full - we discovered it was actually a 40Mb drive so created another 8Mb drive - I bet they are still using it! ;-)

I know, but I'm an old school guy ;)

The tool "MSConfig" of microsoft already built into Windows is very useful for disable services and delete programs loaded at windows startup.

Defragging is freeand always speeds things up if you havent done it for a while.

And (obviously) more RAM. Which is every bit as cheap as software these days!

thanks to all for your really helpful answers. 

The defrag has not had such a huge effect. But i will also try the MSConfig as the booting takes really long.

Thank you everyone, Alex