Download a rom from a website, XDA forum is a good place to look. They usually give you information on how to install, it's very easy.
Here's the site:
Android Forums & Windows Phone Discussion @ xda-developers
Just type HTC Inspire Gingerbread ROM into their search box and find one that fits what you want.
Well its not as easy as downloading a rom and following the instructions in the rom thread as MassacrMan states.
He leaves out a good bit of the steps.
It first involves rooting your phone. Download the kit from this thread
[TOOL] HTC Inspire/DHD Hack Kit V12.2 (UPDATED ON 08-11-2011) - xda-developers As the instructions say read the EFFEN MANUAL
Once its rooted it has a different recovery mode installed. You then want to find a Sense 3.0 rom in this part of the inspire 4g forums
Inspire 4G Android Development - xda-developers
I use Virtous Unity for a sense 3.0 rom.
So folks that respond that its easy to flash a rom. Its not if you don't know anything about it. I came from the Samsung Captivate and that was alot easier then the inspire to do things with. Even though 2 hrs after I had my phone it was rooted and rom'd it still wasn't as easy as MassacrMan makes it out.
When I first got my Captivate I read those forums for several days before I went and flashed. I made sure I knew how to get out of trouble. I made sure I could flash back to stock if anything broke. I read about all the issues that people had with the phone.
You are going to want to download the HTC Drivers for the phone if you have not already.
http://dl4.htc.com/setup_3.0.5557.exe Run that and the first part of the install is the drivers. Once that is installed you will then be asked to install sync. Cancel here. Now you can plug your phone into the comp and mount it and copy stuff to and from it.
If you need any more help then feel free to pm me. This should get you started on what you need to get started.
BUT I STRESS READ READ READ AND THEN READ READ READ AGAIN.
If you have questions search first with google. If nothing it returned then search on XDA if nothing is returned make a post but state you have searched and was not able to find anything similar. There probably is something similar on there but its so vast its hard to find.