I know what it feels... trying to save your old Pc. You didn't mention what operating system you use, but there are 2 ways to clean your Pc and make it fast.
1.- When a Pc works slow, many times is the operating system. So, a quick answer to eradicate your problem is formating your hard disk. To eliminate everything you have into your computer: your files, programs, including your sick operating system.
If you have a WindowsXP installer disc (Professional is better than home edition), put it into your Cd-rom device before your pc starts, hit one key and after it loads, follow the steps: Choose install WindowsXP (don't choose fix), accept the terms and choose install a new copy. Currently you have a partition with some operanting system. A partition, if you don't know, is the space in your hard disk that you can use, but after you set a partition, you need to "prepare it" with a format in order to get it ready to be used.
Because you have an existing partition, when you choose install over the current, the installer will give you the follow options: To delete the current partition, or simply, continue and format the current partition. The difference between those options is: if you delete your current partition, you'll erase everything in your hard disk, you make it virgin (it's a suppose). If you apply a format, you will delete everything too, but only into the space delimited by the partition. It's something like to clean your house (format), or to demolish your house and build it again (delete partition).
I encourage you to delete your partition. Next the screen will give you the option to create a new partition and how much space you want to use. Choose the full an create the partition (Maybe the system requires from you 8 megabytes, there's no problem). After you created it, apply a format modality NTFS. Don't use a quickly format, the normal is better. After format ends, the installer will do the rest of the task, just keep an eye on it.
No matter what operating system you used, if you want to install WindowsXP, the steps before is useful.
2.- If you don't want to lose your information, and the idea to delete everything scares you, I suggest to you a full maintenance of software. There are many applications you can download from internet, like TuneUp, or some companies sell programs for that, like Norton Systemworks. You need to clean the temporary files, backups, cookies, the zero-byte files, clean and optimize your physical memory and, more important, to clean and optimize your windows registry, in others. After use the program for clean everything, update your antivirus. You can download some application for eliminate spy files. Some spyware like "Search and Destroy". I recommend you to run the defragmenter, right-clic on mouse over your local disk, properties, tools and defrag.
With all those steps, your Pc should be better than the beginnig.
Questions? send me an email to isai_201@yahoo.com.mx... Good luck!