Well, there are many things that could slow down your computer from running in its normal speed.
Like large temp files, large desktop files, low space in C: drive, viruses, anti-virus software, network problems, or just simply you don't have the best configuration to run your PC smooth.
2.For those who haven't used task manager before, press alt+ctrl+delete (or) right click on your task bar and choose task manager to initiate this program. Then go to "Processes" tab and sort the programs by clicking CPU. See what program runs in max CPU usage. If you are suspicious(you have to be absolutely sure of this, because if you kill any important system program then your PC will shutdown) of this program then try killing that by clicking "End program".
3.Then, it could be some virus that may be causing the slow down. So you better get a good anti virus software and do a full scan of your computer.
4.If you have the habit of placing all the download files in the desktop and just leaving it there, then it could also be one of the factors for your computer slowing down, so try removing unwanted files from the desktop and place it in other drives.
5.Check for the free space available on your C:drive. Maintain atleast 1GB of free space for page files and system temp files.
6.Also check for fragmentation in the drives. Right click the C:drive and choose "Properties", "Tools" then "Defragment now". Analyze the drive and defragment if it is fragmented. This happens when you often install large applications or delete or uninstall applications.
7.Go to start and type "msconfig" in Run(for XP) and go to "startups". Check for unwanted programs that are starting when the windows starts and remove it(You have to restart your computer for this to take effect).
8.While you browse through the internet cache files, temp files will be loaded into your computer and after some time it will pileup and occupy unwanted space. So to clean this up you can use free softwares available in the market. Do a google search or you may try this one CCLEANER.
One last thing, if your OS was installed a long time ago then it could also be one of the reasons for slow down of your computer. As you install and uninstall software registry files will still be in your PC even if you uninstall the entire program. There are many registry cleaners in the market, but I'm not a big fan of this. I'd tried some softwares , some worked great in XP but not in Windows7 and I had to reinstall my OS to make it work again. So if you want to take the risk, then do a backup of your registry files and do the cleanup. But if you ask me, it doesn't worth it. Just do a reinstall of the OS like every 1yr(more or less) and again only if you think you need to.