Are you talking physical or logical disks?
Physical disks can as far as I know only be joined by using dynamic disks in Windows2k but the question is more related to why you need to do this?
If you are talking logical disks (partitions) you may use
* Ghost for the primary partition, backup to save all data and Fdisk to merge the partitions (recreate), or
* Backup of all data, Fdisk top merge partitions and a reinstallation with later restore, or
* Backup and Partition Magic
In any case it is not that a trivial task