There are many reasons a computer can be slower than when it was new. Windows computers tend to get slower over time as programs are added and removed and changes are made to the software and operating system. You can maintain it as much as you like but it always will get a little slower as its used. Think of it as a car that you tune up over and over but it never runs like it did when it was new. You can perform various maintenance procedures like cleaning up unnecessary files such as temp files and log files. Windows has a built in disk cleanup utility or you can use a third party application like the free CCleaner which does a better job.
One of the main reasons for computer slowdowns is caused by spyware infections that you get from downloading things from the internet that you may or may not even know were being installed. There are many free anti Spyware\Malware programs that you can download and install to scan your system for infections.