Is your firewall activated in Windows? Maybe your tools shut down some of the necessary ports while cleaning up the PC, and didn't reopen them.

Have you tried using FTP to download Firefox? The FTP address is:
ftp.mozilla.org /pub/mozilla.org/firefox/releases/1.5.0.5/win32/en-US/Firefox Setup 1.5.0.5.exe

If worst comes to worst, you can download Firefox on the machine you are currently running, and copy it over to the other through your LAN or via sneakernet (burnt CD/thumbdrive/etc).