BTW, do you know that Windows eats up a lot of HDD space in System Volume Information folder?
Besides that, trojans like to hide in there sometimes.

Do a full system scan with all kind antivirus,antispyware, rootkit etc before you install WOW.
An article shows that most trojan created lately is targeted at WOW accounts.

close ur background programs such as the antivirus in ur system tray. then go to control panel, system ---> adjust the size of ur virtual memory. decreasing it will increase ur hdd space, especially if ur physical memory is low.

this is only a temporary solution. my advice is to backup files that are not frequently accessed into cd-roms or get another hdd to migrate data over. also, u should be uninstalling unnecessary programs that you have installed. say bye bye to 'hello kitty island adventure!'

btw, the error msg says that u only have 3.6g available? i doubt that the solution i just gave will do much good because the patches itself will eat up another big chunk of ur hdd space.