I forgot my screen lock password - HTC Desire Cell Phone

First im really sorry about that! to fix the issue you must do a hard button reset. if you go to http://www.knowyourcell.com/htc/htc-desire/desire-guides/443117/how_to_hard_reset_the_htc_desire.html
it provides the steps as follows:

Please note that all data will be deleted from the HTC Desire as a result of this procedure. This process does NOT delete data stored on the microSD card. We recommend backing up all of your personal data before proceeding.

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 Downfor NO

If you selected YES, all data including third-party applications will be deleted from the HTC Desire. Once the wipe is complete, the phone will reboot to its factory fresh state.

i am 100 % sure this will fixya for sure!!