Windows 7: Skip Office Activation on Setup and have it as a trial

I've obtained: Office Professional Plus 2010 with Service Pack 1 (x86 and x64) - DVD (English) from MSDN.
I wanted to install it first before inserting the product key legally, similar to a trial setup.

Sorry, you can not do that. There are only two ways to have Office 2010 in trial mode legally:

1. Special trial version, which CAN NOT be converted to full, even with a valid key.
2. Volume version of Office 2010 available only to corporate clients.

If you have obtained your iso legally from MSDN, you can legally get full key from there.

I have the key, but I'm trying to install it on my friends PC, to let him try it, he'll buy his own key afterwards.

Those versions you are thinking about, which have both trial and a possibility of a full with a key, they come only with NEW PCs and are OEM versions. They can only be obtained with new pc or by corporate clients.

In case of your friend - install special trial version, activate it with trial key (you will get it from that page). After 60 days it will deactivate itself, and you have to uninstall it. This version won't work after trial even with legal full key.

Once received legal full key you friend would have to delete trial version and reinstall retail (yes, I know that is not cool).

That is all because:
Retail version does not have trial mode and trial version from MS website can not be turned into a full one. And OEM versions are not available for legal download to general public.