Just bought the 100GB version of this external hard drive:
http://www.seagate.com/products/retail/portable

It's formatted Fat32. I'm not finding differences that mean anything to me here (http://www.ntfs.com/ntfs_vs_fat.htm), or at other comparison websites.

Is there a benefit to my reformatting it as NTFS?

Thanks!

The biggest advantage to using NTFS is security !
FAT only really has security as an after thought.
The other advantages are in;

Volume size, max 32 Gb for FAT 2Tb for NTFS
Performance
Fault tolerance
Economy
Max file size 4Gb Max on FAT


So don't delay upgrade today!!

Also you don't need to reformat, unless you really want to.
You can use the convert (http://www.microsoft.com/technet/prodtechnol/winxppro/maintain/convertfat.mspx) command via a cmd prompt
Full explaination here. (http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/convert.mspx?mfr=true)

On that very link, you'll see that NTFS performs better on larger volumes, while FAT32 performs better on smaller volumes.

Also, and you may not care about these, but with NTFS comes file level security and the ability to compress files to save space (although at a cost to performance).

Thanks, Guys - that was quick and easy!

the reason it may have been formatted as FAT is because, as an external, it can be moved and connected to other PCs. By using FAT, there would be no problem connecting it to a Win 9x PC.