You'll need to reformat it and possibly restore the MBR, depending on the method you used to make the bootable drive. I forget exactly how in Windows but the reformatting bit can probably be done by right clicking on the drive in your file manager, or possibly even accepting an automated dialog from Windows on plugging it in if I recall correctly.
Restoring the MBR is a bit more complicated and something you may well not have to do. Reformat first and do this only if that doesn't work. I actually had to do this manually from the command line once, but now it seems there's a graphical version of TestDisk for Windows to deal with it. Here's a guide on how to use it (also quoted below in case it should ever disappear) and a link to the program itself (you want the one just labelled "Windows" probably).
1) Download the Test Disk utility
2) Connected the USB Drive to my laptop. (windows kindly offered to
reformat the drive as the previous operation had failed... I obviously
3) Ran the TestDisk program and select Analyse disk... takes about
4) The analysis results indicated the data was intact but the MBR (aka
Master Boot Record) had been deleted.
The TestDisk message went something like "the extrapolated boot record
does not match the current boot record"
5) Selected "Rebuild MBR"
6) Selected "Write Boot"
7) Selected "list" (which lists files and folders)
8) Hey Presto!! Files and folders were listed!
EDIT: Couple extra comments here. First, if you can't reformat the drive you may have to restore the MBR first. I know Linux generally won't even show the drive in the GUI if the MBR is messed up. I've...done this a few times before. Second, if you want to create a Windows 7 boot disc Microsoft offers its own tool. I don't know UltraISO - maybe it works just as well for making bootable flash drives as bootable DVDs. Not all programs do though. I also have used Unetbootin to make Windows flash drives with great success in the past.