Iobit.com - Driver Booster, Advanced System Care, Smart Defrag, and Malware Defender (all come in free or paid versions with more "support" built in).
Piriform.com - CCleaner
Windows Update, if you have a Windows computer.
Use all of these on the default settings only. Change NOTHING. You may need to reboot once in a while.
Run CCleaner three times - once to clean the registry, once on Cleaner, and once on Tools (after defragmenting) to wipe the unused portion of the hard drive of all deleted files - use the "wipe three times option" - this stops the computer from looking at everything in that area to make sure that it is marked as deleted or not in use. Otherwise, the computer double checks the unused area because it has to make sure that "those files" really are marked as deleted or not in use.
This means that you don't have to reinstall any software and reactivate it - but it is not going to speed things up quite as much as a clean install of the operating system & installing/reinstalling only the programs that you KNOW you use on a regular basis.
A clean install of the factory image, running updates on the operating system, the drivers, installing & activating any extra software, removing anything that you don't want, updating all the drivers, running CCleaner, Advanced System Care, Malware Defender, Driver Booster, & so forth takes LONGER to do - and you probably still want to end with Smart Defrag & CCleaner to wipe the "empty" portion of the hard drive to make sure that the computer isn't spending time double checking anything written to that area in the past....
BUT it does end up doing a slightly better job. I'd say that it depends on how much time you want to spend looking for all the installation disks or downloading & installing programs, copying your personal files, photos, & music to a backup system, and the payoff on how much better that might work for you.
If I'm replacing the hard drive in a computer - I've started using cloning software to copy a "clean install" that I did & saved on a 60gb hard drive that has all my software on it plus the cloning software - then running Windows Update & Driver Booster to get the larger hard drive that I will really be using completely up-to-date. It's a time saver, for me.
I *like* playing with my computer and getting it tweaked just right - but once I have it "just right", I see no reason to reinvent the wheel if the hard drive gets infected, fails, or I find one with a faster coprocessor on it for sale on eBay (I've upgraded that way several times over the last three years - but since I can't sell the "spare" I'm not updating again - the difference between 2.5GHz and 2.6GHz is not so huge that it's tempting me beyond my *budgetary* limits).
I have a GOOD computer and it meets my needs very well - my embroidery machine software is not going to get a Windows 8 upgrade/update - so getting a NEWER computer would cost me more money than just the new computer - I'd need a new embroidery machine, too. Possibly a whole new suite of embroidery software...depending on what came with the new(er) embroidery machine....