Memory defrag...? Defrag is for hard disks.
And even if you clean your registry each week and defrag your comp, its still better to shutdown your computer than hibernate it.
You may not have any problems with it getting slow, but try to hold it a whole week working, you'll notice a difference in performances.
For example, after like 5 games on Warcraft 3 with my P3 787MHz, the games will take some more seconds to load. the game will start to lag.