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.

1. Turn the power off. If your HTC Desire is frozen, pull the battery out and reinsert it
2. Hold the Volume Down button
3. Press and release the Power button
4. You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
5. Select Clear Storage by pressing the Volume Down button
6. Press and release the Power button
7. Now simply confirm your decision: Volume Up for YES and Volume Down for NO

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.

1. Turn the power off. If your HTC Desire is frozen, pull the battery out and reinsert it
2. Hold the Volume Down button
3. Press and release the Power button
4. You are now presented with a menu that allows for Fastbook, Recovery, Clear Storage, and Simlock
5. Select Recovery
6. On the next screen press Volume Up and Power
7. You should then see the "Android System Recovery"
8. Select the 'Wipe data/factory reset' option
9. You may need to confirm your choice (i don't want to wipe the device i have so not 100% sure)