It depends on how it was formatted. PC disks can be formatted in several different ways - FAT, FAT32, and NTFS. Disks under Win95/98 will be FAT or FAT32. Disks under NT/Win2000/WinXP will typically be NTFS.
OSX should be able to see a FAT or FAT32 disk without any problem. Disks formatted as NTFS will not be readable by OSX (nor in OS9, afaik).
However, all is not lost as there are a couple of freeware utilities out there for OSX that will enable you to read an NTFS disk. One of them is called "NTFS on OSX" or something.