You can fix most things yourself!

Ok your computer running like a slug. There are two reasons. Hardware or software.

Hardware: the hard disk slows down just due to age (the disk grease dries out eventually) 2 to 50 years depending on temperature and vibration.
The surface coating of the disk rubbing off. The space left on the hard disk and the order the files are in.

Software: Updates, outdates, junk files, cookies, spy ware, old applications, to many things running in the background.

To the average computer user all these things hidden from you. Some are for safety, some are "you do not need to know" and some are spying on you.

I have a list of software by people who have you interest at heart. I will provide a link and a brief detail on what they are good for.

Spybot Search and Destroy

This programmer looks for spyware that installs it self without you knowing. There is about 350,000 different spyware codes on the internet. If you have problems use support or forum pages on the site.
Go to the download page, pick the latest download, download it from one of his servers. Install it on your computer, it asks you do several steps on the first install. Allow it to update and then do a full scan. This can take a long time (walk to the shops get something healthy and walk home). Be aware you may have to re scan several times and reboot the computer to remove persistent software.
Once you have used this software give them a donation so they can eat to. There are no adds on the site to give you confidence in their motives.

Ad-Aware by Lavasoft

Ad-Aware is much like Spybot and complements it very well. They have a free version whish is a good starting point. This program has a different focus to Spybot. Ad-Aware targets Malware which is malicious self installing software. I suggest you install it update it and run it.

CCleaner

CCleaner is a freeware system optimization, privacy and cleaning tool.
It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast.

PC Decrapifier

This is a good little program for removing the crappy software that comes "free" (well eventually it will ask for money) with a new computer. Things like Document to PDF converters that after a couple of uses sticks a logo in your PDF. Anti virus software that sort of works for 90 days (BUT only if you register first) PC Decrapifier helps. You can delete it after you have used it as it will only be used occasionally and it would be best getting a current version.