X51R1 Install from external drive?

Hey all. Probably a stupid question, but here is the background:

I bought the X51 R1 when it first came out, and it has been a brilliant little machine ever since. The only thing which went down was the cd/dvdrw and that was no problem as I was more than happy to use an external drive.

Last week, however, the hard drive died, so I have replaced it with a new one. Now of course, I need to install windows. and having no cd/dvdrw in the X51, I am wondering how to do this, as having ensured the machine is set to boot from the external cd/dvdrw, it simply won't! (I have a win 7 disk in the external drive).

Does anyone have any suggestions please?