... there're numerous things that can slow a PC down. First every time you install a new program, it takes resources on your hard drive, extra entries in your registry, and all too often new programs to be loaded at startup. These slow the booting process and take up RAM. Windows updates and updates from Adobe, your antivirus software, and other programs like Firefox can cause software to grow and often replace existing files which can fragment the hard drive. Hard drives can develop bad sectors, causing the assignment or alternates; and this causes extra seeking which slows things down.
If you watch out for started programs using MSCONFIG's Startup tab and be sure Defrag runs periodically, that will go a long way to reducing this slowdown. At least once a year you should also Chkdsk and let it check for and attempt repair of bad sectors on your hard drive. Also sometimes dust can build up and cause excess heating inside the computer. This can slow things, sometimes cause "hicups" so at least once a year you should clean out the dust.