Making Windows Faster is not hard to do. You just need a little bit of time, and don't have to spend much money or any money in most cases since most of the software like disk cleanup, disk defrag, and msconfig startup manager comes bundled for free with Windows, and there are a lot of free and cheaply price third party software for making Windows faster which you can download in Windows.
If I'm using a old computer which was used by many people, I would scan it for Virus, spyware, and malware programs, since all these types of software can really slow down your computer since they make your computer unsafe while slowing it down because they run in the back ground of the computer. AVG, Avast, Microsoft Security Essentials are free antiviruses which you can use to scan your computer for viruses. Malwarbytes, SuperAntiSpyware, and Spybot are free antispyware programs which you can use to scan your hard drive for viruses.
Overtime when you use your computer, it is filled up with junk files, and your hard drive becomes fragmented. I recommend doing a disk cleanup, and disk defrag on Windows. CCleaner is a good disk cleanup program, and Auslogic disk defrag are great free software for maintaining your hard drive performance on a regular basis. CCleaner also has a program uninstaller, registry cleaner, and startup manager which you can use to improve the performance of Windows.
I recommend Advanced SystemCare with Antivirus 2013 to keep your computer fast and secure quickly, and easily!
Startup programs in my experience are one of the main reasons for slow computer startups since when computer programs start up they use RAM and CPU system resources which can be better use to run more important programs like your web browser, computer game, MS Word, etc rather then a instant messaging program which starts up automatically, but rarely uses.
Use msconfig in Windows to make Windows startup and perform faster