I had the same problem as Martin32 and Pete_the_Meat above in that iplayer would stream but clicking "Download to Computer" caused my browser (IE 7) to crash. 4OD installed OK but would not load (i.e. some HD activity after clicking shortcut and then nothing).
I traced the problem to the Kontiki service (KService) not starting properly. To confirm whether this is the case for you, click start > run. Then type "services.msc". Check the status column next to KService. If it's blank rather than "Started" then try starting the service manually by clicking the play button in the toolbar. If you get:
! Could not start the KService service on Local Computer
Error 0x80004005: Unspecified error
as Martin32, Peat the Meat and myself did then you are probably having the same problem. So, here's the solution:
1. Uninstall Iplayer Download Manager and/or 4OD using Add/Remove programs.
2. Uninstall Kontiki by running KClean.exe http://static.sky.com/kclean/KClean.exe
3. Then delete the remnants of the Kontiki folder in Program Files if it's still there.
4. Check the Services list again. KService should no longer be present.
5. Reinstall Download Manager and/or 4OD.
6. Check Services list. KService should now be present and "Started".
7. Both programs should now work!
Note: Before doing the above, I eliminated my router firewall as the problem (as suggested by Pete the Meat) by disabling it temporarily and trying to start KService. It still did not start so I did some more research and came up with above fix. Hope it helps.