the bad part comes first - i didn't find a disassembly guide for the htc desire. but if you are careful - you can do it without a guide.
the iportant part is cleaning get some isopropyl alcohol with more than 90% and a soft paintbrush, disassemble the phone and clean the mainboard and all connectors with the alcohol/paintbrush.
the white/greenish residue must be removed.
let the phone dry (don't use a hair dryer)
reassemble it and try to power it on. sometimes the battery gets discharged due the shorted parts so charging might be necessary after the reassembling