Trying to factory reset, but it's not listed at all as an option
This is a quite often asked question -- so please see the "Linked" and "Related" sections for more answers.

A good place to check for answers concerning factory reset is always hard-reset.com. At Samsung I5800 Galaxy 3 Android hard reset, they write how it can be done for your phone. Basically, it involves the following steps:

Using hardware buttons only


Press MIDDLE of the VOLUME + MENU (middle key) + POWER ON for 1-2 sec
Release Power. Recovery menu appears.
Select Wipe Data/ Factory reset with volume down, then press MENU
select Yes - delete all user...
press MENU or POWER button


They further recommend: to hard reset i5800 Galaxy 3 battery must be fully 100% charged.

Alternative approach

As you still can turn on your phone normally, there's a second option available:


open up yout telephone app (dialer)
dial *2767*3855#
follow the instructions


This is not a real phone call, but rather a "magic number" (shortcut to system functions).

Last word: Be aware that with a factory-reset all your manually installed apps and all your data on internal/phone memory are lost, so backup what you need before! I guess you knew that, but just in case ;)