Did you format it for Mac OS X (HFS+ Journaled)? That is unreadable by Windows.
You can either buy a utility to read/write HFS+ in Windows. Such as Paragon HFS+ or MediaFour Macdrive. Which is what you will have to do if you use the drive for Time Machine.
The free option is to format it using a filesystem natively supported by Windows and Mac OS X. Either use FAT32 or ExFAT. I would use ExFAT due to the 4GB file size limitation of FAT32. This breaks Time Machine support (unless Apple changed that). You will likely have to use Disk Utility in Mac OS X to do the format. Window often balks when you try to format a GPT disk originally formatted by a Mac.