When comparing with older versions… MS Window 7 does a good job at storing your data in a way thus fragmentation is somewhat, low. But still if you install/uninstall a lot of programs or copy paste large amount of files frequently then perhaps you could see some reasonable performance gain if you just defrag your hard drive.
But figuring that all by yourself is impossible so you can use a utility like O&O SpeedCheck which is a freeware that automatically checks whether you should or should not defrag your data and lets you know about it.
Remember, SpeedCheck does not defrag your data but only notify you whether you should or should not.
*. No installation needed. Just download the file and double click on it and you’re good to go. So it’s also a portable app as well.
*. Supports Windows XP, Vista and Windows 7.
This utility basically uses data that are fragmented/de-framented in a simulation and compare the time-frame difference between them (after doing read-tests) and give you a calculation of how much performance increase could be achieved.
Interestingly though no actual files on your HDD are read… the software only uses a “simulation” of both fragmented/defragmented files (created by itself) for figuring out how the OS would deal with them in general terms.
If interested you can get it from this O&O SpeedCheck page.