If You want to downgrade Desire Hboot 1.03, then using revolutionary to s-off, please follow the instructions below.
1. Download tools package. THIS LINK
2. Download this HTC Desire ROM (RUU)
3. For branded phones create GoldCard (How to)
4. Extract the tools any where at your computer.
5. Run RUU and wait to appear Android rom update utility screen. Don’t hit Next.
6. Go to Temp directory on C drive (Press Window+R and type %TEMP%, then hit enter) on your computer and find folder that RUU extracted in it.
7. Copy the rom.zip and paste it to sdcard and rename it to PB99IMG.zip
8. Connect your phone to computer via USB as charge only and enable USB debugging and Unknown Sources.
9. Run DOWNGRADER on tools folder that you downloaded and extracted it. If it gives any error try running it as Administrator.
10. Follow the instructions on screen (This may take a minute or tow)
11. Your phone will be reset, wait for detect and check the Update Package.
12. After checking update package, press vol-up to begin update process, IN FIRST TIME AFTER PRESSING VOL-UP YOUR PHONE WILL RESET AND DO NOTHING, YOU SHOULD DO AGAIN FOR STEPS 11,12
The process will flash your recovery partition back to stock
This process will wipe all the data from your phone
The downgrade installs an official stock HTC ROM with Hboot1.02 (You can s-off with revolutionary and after it You can flash any version of Hboot)
Your sdcard should be fat32 formatted