You must have reloaded the XP OS. The place to look is Right-click My Computer->Hardware->Device Manager->modem or bluetooth.
If nothing is there, it will mean when you reload bluetooth device is not detected and hence not installed. To switch the physical device on without using the tablet button, you need to press F12 at the ACER reboot screen and do it in the BIOS and reboot again.
That said, the XP OS will only load the MS bluetooth device driver which is not too useful.
The easier way is to goto the ACER tech bulletin and download their bluetooth driver sets (which actually is a WIDCOMM one) which is the pre-installed one which came with the tablet.