No. USB will not work to boot windows from. If your motherboard has SATA interface the best option would be to purchase an external hard drive enclosure that has E-SATA and it will come with the (usually) required bracket and connectors. Do you have an extra hard drive already?
The eSATA option is great - someone mentioned that you need to purchase an enclosure with SATA or eSATA option. The isn't necessarily required if you have a fairly modern ext HD. My WD MyBook Home has USB 2.0, Firewire and eSATA.
The next question is whether your PC has an eSATA port - unless you have a fairly new PC/Motherboard, you may need an upgrade on that side.
I got a question on that: I'd like to install Windows 7 on my external HDD. It has an eSATA port, but my laptop hasn't - so I figured it would be best to 'install' one by an CardExpress-eSATA adaptor. My question is: Will installing Windows 7 work like this? (I know it doesn't work with USB.)
Which adaptor should i buy in order to make sure it works? Nobody seems to be able to help me... Or is there another way?
Provided the motherboard supports it (which any board made in the last 5-6 years should) booting Windows 7 from a USB connected drive will work, it's just not practical as an everyday solution for running Windows, given that it's bandwidth capped at 60 MBps.
Any eSATA adapter should work, as Jonmor said above, just make sure it is connected via the ExpressCard slot instead of the older CardBus slot.