Singh,
Where to begin... there are so many reasons a machine runs slow and I only know a handfull, running the MSconfig is one way to stop all the unnessary tasks from loading at startup like Dipa said. this will allow it to load faster and not take up memory, but with any thing you do to a system becareful what you disable.
You can run defrag on your hard drive to help move fragmented files for faster access, and periodically re-run defrag or at least do an anylize on the drive to see if it is needed. (depends on how often you download and erase large files and or software)
have you experience any thing happening that you think should not, meaning virus like.. I used to run Norton system works alot to fix missing links or dll files even to check time date stamps on files.
If you are not sure what you want to disable, try this. once the computer is up and running, go to the system tray and look for any applications you dont really want or need and close them / stop them from running. once you have closed all the things you want and your think the maching is running a little better. do what Diap said and open MSconfig. sort all the files by running or stopped... look for the stopped files with check marks, and uncheck them if you are not running it you dont want it to load in startup, their may be a printer driver that is not running but leave that one.
just take your time and you can always re run msconfig to stop more from loading.
Also look in your startup folder, remove as nessassary.
One more thing is my daughter loves to all all kinds of toolbars to IE. I am forever closing or removing these, you might look at those as well.
I am sure there are a hunderd more way to speed up proformance just becareful what you remove and or delete.
Let us know what you find out and how you do
Don