@Simon
First of all you need to replace the battery,u can get a new one
Add another 512MB memory stick,i think the 120gb Hard disk is not bad
as it were,u can get an external one
costs as follows:
Battery: 6500
Ram 512mb is 1850
External harddisk e.g 320GB range is 6500 depends on make and ofcourse quality
As for the harddisk looking full the reason is that you have bugs in your
system that clog the registry and multiply making your system unusable.
Drop me a line
solutions.clearvista@gmail.com i will sort you out pronto
Teato5