The O2 Xda brand is a range of O2-branded Windows Mobile PDA phones, produced by multiple manufacturers (mainly HTC, Quanta and Erima). The first model was released in June 2002. The "X" represents convergence of voice and information/data within one product; the "da" stands for "digital assistant", as in PDA.
You will need to know the exact model or series of 02 XDA that you have, as the reset technique is different for nearly all XDA Units. This information is usually located on the rear case of the unit, or inside the battery compartment.
Once you have this information, just select your unit from the O2 XDA Model list, and follow the reset instructions exactly.