HTC - Root & Flash ROM

Stage 1: Get Amon_RA into the Desire
1. Download Unrevoked.  Link here
2. Download Amon_RA  Link here
3. Install HTC and HBOOT driver.
4. Run reflash.exe as Administrator (DO NOT connect your Desire to computer at the moment).
5. At Unrevoked, File>>Custom Recovery>>Choose recovery-RA-desire-v1.7.0.1-R5 (the Amon_RA file you download in step 2).
6. Connect your Desire (USB debug mode on, etc).
7, The rest is fully automatic.  :)

Stage 2: Partition you SD Card
1. Enter into Recovery mode.  You will see the options below:
    - Reboot system Now
    - USB-MS toggle
    - Backup/restore
    - Flash
    - Wipe
    - Mount
    - Partition sdcard
    - Other
    - Power off
2. Under Partition sdcard, you will see:
       - Partition SD
       - Repair SD:ext
       - SD: ext2 to ext3
       - SD: ext3 to ext4
3. Use Partition SD to make the ext2, swap, and the remainder will be auto partitioned to be FAT32.
4. press back to return to step 2, select SD:ext2 to ext3 to upgrade the ext2 you created in step 3 into ext3.  Do SD:ext3 to ext4 if you want to upgrade further to ext4.  But I only upgrade to ext3 only coz i read somewhere ext4 is not necessary.
5. Reboot system now, it will boot with all the partitions now.

Note:  If you want to flash rom, you can use the Wipe function to wipe all ext3 partition, cache, etc clean, but the partition remain intact.

Hope this mini guide helps. For more details, should refer to this awesome guide!