如何通過外接CD-ROM將x61的VISTA換成xp pro?

問題如下，試了好幾次，都是這個問題。

Basically, I got my X61s with vista basic, and I want to replace vista with XP pro. What I did are:

1. Connect it with an external DVD rom through USB port.
2. Login to BIOS setup and change the harddrive to compatibility mode.
3. Boot from external DVD drive with XP pro CD software
4. Get the C drive formated and start loading XP files.

So far so good until this step, however,

5. After the startup files were loaded, the system is rebooting and installation is continued.
6. When the system rebooted for the next step installation, it can not find the required files from CD software disc, it prompts me to input the path, it looks like it doesn't know what the drive letter for this external dvd drive, I tried a:, d:, e:, etc. none of them works. and the installation can not be continued. This is so weird to me.

哪位高手給些建議，應怎樣解決這個問題？

多謝！