mandriva ISOs are hybrid ISOs which means that if you copy an ISO to any device(including USB) in a low-level way(RAW) it will be able to boot from that.In a Linux system you can use the 'dd' command-which is an utility for copying data from a device A(or path) to a device B in unformatted(Low-level) way.If you are gonna use it firstly be sure u know to use it well cause is pretty dangerous to lose whole disk due to a single typing error.
But,you can download the 'seed' program form the Mandriva site for Windows( as u said that Win 7 is what u use).
go to:->
http://www2.mandriva.com/downloads/?p=seed
P.S I have already twice used 'seed'(linux edition) to make USB Mandriva ISOs and successfully installed on my PC