Something I didn't check yesterday, but are you doing the download via MapSource, i.e. Help>Check for Sofware Update... or using the
direct download from Garmin.com? I ask because the second is a much more reliable approach than the former (as long as you click the "Save" rather than "Run" button on the download dialog). Once downloaded, just double-click the executable to install the new version of MapSource.
Also, if you want to check the download performance of your system (from ISP through to your PC) to verify that this is a problem unique to the MapSource download rather than a general performance problem, then I suggest using one of the many speed testers available such as
Kbps. Just click on the "Autorun Speed Test" option in the main menu and it will give you the lowdown on what sort of up download and upload speeds you should be able to achieve on your connection.
Finally, I would
not recommend disabling any Firewall you may have installed on your PC unless you are sat behind a hardware firewall.