I want to help you so I might not quit...i have the same phone so I would like to help you out, these are basics but atleast you can try :
1. try a different battery even if it cost you (n93 is more expensive the battery)
2. try your original battery on a different phone that uses the same battery to make sure that battery is not DOA atleast or faulty.
3. try charging on a different outlet.
4. if you are using a NA adapter try different one it might not making good contact, hence wont charge.
5. if you are using it on a windows xp turn off your pc, plug your phone turn your PC on so they will start at the same time as windows, some hardwares requires this even on XP.
6. try a different charger even the old one just use the adapter plug or use a car charger if you have one.
7. if all else fails check with iunlock.
8. keep us posted alot of people want to help you here even those who doesnt have the same phone you have..