Everytime I try to download ANYTHING (including Limewire) on to my Mac OS X, it says "cannot create file" in a popup window. Can anyone help me? Thanks a bunch!

I am using Safari and downloading a .dmg file. I think I am up to date with my upgrades.

Here are my system specs:
Machine Name: Mac mini
Machine Model: PowerMac10,1
CPU Type: PowerPC G4 (1.2)
Number Of CPUs: 1
CPU Speed: 1.25 GHz
L2 Cache (per CPU): 512 KB
Memory: 256 MB
Bus Speed: 167 MHz
Boot ROM Version: 4.8.9f