Hi
You have not said in what circumstances it crashes. One common cause is a lack of memory. The obvious solution is to reboot the phone. If the power button does not respond to turn it off, take the battery out.
If you can, install History Eraser by InfolifeLLC (free) from the playstore/market
Use it to clear all caches (this wont erase your data) It should free up some space. If you are still having trouble I suggest you register on the forums and post another question in the forum for the HTC Wildfire and give as much information as possible:
error messages, what you are doing when it crashes, how much memory left etc.
Roger