To Root your HTC Desire using unrEVOked3, follow the instructions below,when visiting the links make sure that you follow the instructions to the letter.

First of all HTC Desire Portal will not be held responsible if you brick your HTC, proceed at your own risk. Before starting to root make sure that you back up your contents.

Before proceeding make sure that your Battery is fully charged, if you try rooting without a fully charged battery it will not complete and could brick your HTC Desire.

1)  You will need to download unrEVOked3 Download

2) Before running unrevoked un-install HTC Sync.

3) You will need to install HBOOT Driver using the instructions here.
It will say that it is for the EVO 4G but works for the HTC Desire.

Next part is how to Root your HTC Desire.

1) You will need to change a setting on your HTC Desire to run the reflash, Settings-> Applications  Development ->make sure that USB Debugging is ticked.

2) Run reflash.

3) Connect your HTC Desire to PC, connection type Charge Only. Follow on screen commands.

3) Wait a couple of minutes, if all goes well your HTC Desire should now be rooted.

4) Once rooted you can re-install HTC Sync.

5) Once you have rooted your HTC Desire you can download Roms, XDA is one of the best places to find a Rom.

Once you have a rooted HTC Desire head over to the Market and install Titanium Backup.

You will be able to use apps which require root access, some Apps on the Market require Root access.