I have a PC with dual-boot, and I have installed Xubuntu (always newest version) and Windows 7. I have a new HD and I would like to be able to access it from both OS. I suppose NTFS would work since Ubuntu supports it, but, I would like to know if there is another format that may be better.
Well Ubuntu does supports other formats better than NTFS but windows only reads NTFS and FAT32 ... you don't actually have any other choices other than NTFS. FAT32 is a bad idea. You shouldn't be able to copy files larger than 4GB into the partition. The maximum file size on FAT32 partitions is (2^32)-1 bytes, or one byte less that 4GB.
So in short NTFS is an easy way to go :)
You can also use Ext2Fsd, which seamlessly enables Ext2/3/4 support in Windows. I used it for a few years and it worked like a charm.
My experience says that it depends... Ubuntu (and most Linuxes out there) supports NTFS through ntfs-3g. I've seen that its performance is a bit lower than native ext2/3/4. If you plan to use Linux most of the time (or you will do most data writing in Linux) then I recommend to format your data partition in ext4 and install Ext2Fsd in Windows.
If you plan to use Windows most of the time, NTFS is the way to go.
NTFS will be your best option for the Dual boot despite other options are better.
Ext2/3/4 is the default filsystem for Ubuntu, and is supported on Windows, but you may need a 3rd party tool to enable as explained previously.
FYI, do suggest to install Windows 1st, next install your Xubuntu/Linux distro, so this way the Grub Loader can manage the boot options upon powering on your system.