Which accident causes windows computer slower?

A computer running Microsoft Windows XP should continue to run at the speed it did when you bought it. They never (or very rarely) do in real life, for a number of reasons. The most obvious is that both the operating system and the applications change as security holes are patched and new features are added.
Unless you are the sort of person who actually benchmarks new PCs, then you won’t really know how much it has slowed down, if at all. (Windows 7 and Vista have reliability and performance monitors that help.

It is certainly true that malware can make a PC run slowly, and you should double-check Norton’s opinion by running an alternative as a one-off test.
But your PC may also have slowed down because it’s running lots of background programs installed by apparently reputable companies such as Adobe, Apple, Google and many others, including Microsoft. Anti-virus software generally cannot protect you from software that you install deliberately.
At this point, you can try to solve the problem either by debugging your current Windows PC or by reinstalling the Windows XP operating system. (You can also not solve the problem by taking a different route, such as buying a new PC or installing Linux. These will replace your current problems with a different set of problems.
And you can choose computer cleaner tool to cleaner up the windows by using upcleaner tool.
UPCleaner tool is the most popular Windows system optimization tool. It comes with five advantages: provides smart optimization schemes and speed up your computer, real-time clear memory to keep your computer at peak performance, find out what make your computer run slower , monitor computer performance directly and easily, More importantly, it is a permanent free use and so on.

One of them being a freeware registry cleaner.
UPCleaner is available for all Windows versions,you can download this computer cleaner tool in here: http://www.upcleaner.net 
Resource: http://blog.upcleaner.net