now and then, only every few months or so. files get fragmented on your disk just through normal use which makes it run slower cos the disk has to do more reads to open files. Defragging puts all the files in a contiguous chunk so they can be opened in 1 read, so your PC runs faster. comprendez?
Depends how much stuff you download/install and how often. Once a month/two months if you don't add much to your hard drive, once a fortnight is good if you are a heavy user (or before/after installing large games). Defragmenting simply moves everything around on your hard drive so that the files are in a coherent order. It also puts together files that have been split up on your hard drive. This essentially makes your pc run faster as everything can load quicker off the hard drive. The defragmenter with Windows is alright, but verrrrry slow If you can, use the speed disk tool on Norton Utilities - is much quicker and lots better.
Overclock your psu. Buy more memory. Any other tweaks you can do with Windows to make it go that ickle bit faster are not really worth it at all.
I've noticed a great improvement in performance of my pc since I went from 512 to 1024, but most things I use are very resource heavy... (at the mo only got 400MB physical free) If Cookee doesn't have at least 512MB, I'd advise to upgrade to that at a minimum.
I very rarely defrag.. and I download loads.. I never really see the benifit of defragin, it just doen't seem to work for me 1GB should really be the min these days.. So much shite takes up too much mem.. Frequency doesn't seem to matter either.. its only really matters for games and stuff.. CPU upgrades seem pointles to me unless ya jumpin up into a new fsb bracket or perhaps goin to a a64.. I've got a 2800+ running at 2.3ghz and tbh it doesn't seem that much faster.. Hard drives matter a canny bit I reckon.. gettin some raptors in a raid setup would be well advisable even though it is pretty expensive.. my m8 installed windows on a raided pair of 74gb raptors in bout 10 mins.. For browsing the net the best thing to upgrade would be ya connection.. If you do decided to upgrade though make sure that u've got a decent psu.. 450 - 550 watts, cos it'd be a bitch if ya got the upgrade done and you didn't have the juice to run the machine.. Sorry bout the long post.. just had to give my 5cents..