Windows XP Professional does not have a specific cap on hard drive space, as it can theoretically handle up to 1.1 exabytes or 500 petabytes per file. However, practical limitations may arise from the BIOS or drive controller capabilities, which depend on the motherboard. Users with larger drives, such as 250 GB or 200 GB, report successful usage, but partitioning the drive is recommended for better organization and potentially improved load times. In some cases, an Ultra ATA/133 PCI adapter card may be necessary to fully utilize larger hard drives.