You should be able to wipe via the recovery however, this will wipe all the data on the device including contacts, apps, pictures and videos.
- Turn the power off. If your HTC Desire is frozen, pull the battery out and reinsert it
- Hold the Volume Down button
- Press and release the Power button
- You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
- Select Recovery
- On the next screen press Volume Up and Power
- You should then see the "Android System Recovery"
- Select the 'Wipe data/factory reset' option
- You may need to confirm your choice (i don't want to wipe the device i have so not 100% sure)