Most people thought that when Windows Phone 7 came out it would be tough to get custom ROM's for it. On the contrary,  XDA member ansar.ath.gr has developed the first Windows Phone 7 ROM's for the HTC HD7. The installation is pretty straightforward and doesn't look to hard for a user to follow if they have flashed RUU's before. Here is a quote from the OP:

Dear forum members,

I successfully created and flashed the following two (2) WP7 ROM nbh images for the HD 7 device branded as Htc Europe.

The first using the imgfs.bin from the Mondrian ROM, not sure of what branded device that can be downloaded from here:

the second using the imgfs.bin from the

RUU_Schubert_hTC_Asia_SEA_WWE_1.57.728.02_Radio_5. 52.09.16_22.33a.50.10_Signed_SCHUBERT_RELEASE.exe

This ROM is branded for South-east Asia (Singtel).

If you wish to learn more about this, then check out the original post, there you will find info on how to install it.