If I set my HDD as first boot how do I boot from DVD? or USB? Won't it just boot from the first boot device it finds?
If it found no boot in DVD should it not then go to the HDD and then Boot?-
Lets face it, for every day use, you're only ever going to need to boot from your system OS. I personally just set the HDD containing my OS as the only bootable device. Now, in those rare cases when you need to boot from USB or CD/DVD, there is always the Boot Menu for that purpose, which you access by pressing F11 during POST (when the BIOS logo shows).
Stu what bios are you running? Maybe thats the issue I am on 1.8
Having checked as I really didn't know (

) I'm actually on 1.9, but never noticed this issue on previous BIOS versions.
Looks like I might be stuck with 2.2T screen on start
If everything else works otherwise, then yes, leave it enabled if it that works. The splash screen won't show if you disable the 'timeout' option, as Lithium466 advises.
However, if you're prepared to dig a little deeper, with Infinity disabled (when your HDD won't boot), try booting from your Windows 7 DVD and seeing if you can get it to repair your current installation.