I'm sure the Express team will be along to help shortly but as most users have no issue updating Express the fault is more likely to be attributable to someone other than Garmin. That's not to say nothing ever goes wrong at Garmins end but there are far more issues that have nothing to do with them.
Express 3.2.4 installed on both my Win7 machines without any problem, have you tried downloading it rather than updating/running it ... then right click the .exe file and select 'Run as administrator'?
Express 3.2.4 requires the Windows Installer 4.5 to be installed on XP. It should have been included in an earlier update to XP before Microsoft discontinued support but it can be installed from http://www.microsoft.com/en-us/download/details.aspx?id=8483
I did as the Garmin customer care guys said and updated to the latest version of the Windows Installer from Microsoft and problem solved! Express installed with no snags. Of course right after that my bluetooth to phone connection quite working!!! It disconnects every time I hit pair mobile device and says sync failed! Worked fine last week..! If it's not one thing it's another it seems.
I also had the problem installing Garmin Express 3.2.4.0 on Windows 7. The reason it wouldn't load was because my virus checker (Sophos) was inhibiting the install because of something called 'Shh/Updater-B'. This is a false positive and Sophos should have updated to ignore it. I forced Sophos to update and Garmin Express installed with no problems!
Yes of course I tried right click run as admin, I even assumed full rights to the pc even though I am administrator.. still no luck. In the log it's still having issues with .NET
If it's a .net issue try the repair tool, see this FAQ https://support.garmin.com/support/searchSupport/case.faces?caseId={c9ab5190-3cd7-11e3-f27b-000000000000}
