i think ur question got ans at the 1st few posts. flashing will erase all your files in phone memory, so backup or move them to storage card b4 you flash.

beside backup, you also need to fo a hard-spl, if you flash a custom rom.

coreplayer is solution to wmp.

HardSPL.

Flashing ROM will wipe out all data in the phone storage, but not the memory card's storage.

PLEASE REMIND THAT HARD-SPL IS NEEDED BEFORE FLASHING ANY 3RD PARTY ROM!!
3rd Party ROM will void warranty for sure and WIPE Phone's Memory while retain memory card (microsd) data. But Software installed will be clear too.

Here is the Video Tutorials (Backing Up & Installing A Rom + Hard_SPL or Stock_SPL)
He[Biovolt] did a great job on this video tutorials. 

If not you can check it out , at our 1st post there . 

Thanks.

if im not mistaken, m'sia x1 also using stock UK rom. not like htc got asean rom.

here is the link for stock roms.

download the tcpmp player and flv plugin, install it then open your flv video with tcpmp player.

http://www.box.net/shared/y7zl5ithqj