I assume the primary partition you want to convert is not currently the active partition.
Normally a "logical drive" can only be created in an "extended partition". You first create the extended partition and within it you can create one or more logical drives.
Not familiar with Partition Magic myself, but PM must be able to do what you want, perhaps you need to convert the primary partition to extended first, then to logical drive?
Otherwise, if you're using XP and the partition you want to convert happens to be empty, and if the partition is not the active partition (XP calls it the System Partition), then you can simply delete the partition, then from the new unpartitioned space, create an extended partition, then create a logical drive within the extended partition. (Not exactly a conversion, but the end result is the same, but you do have to format the new partition again.)
To do this in XP, click Start, Run, type diskmgmt.msc and press Enter.
In the lower half, against the entry for Disk0, you should see your current partitions. Right click the partition and select Delete Partition. confirm the action. That area will become unpartitioned space. right click and select create new partition. Then select extended partition. Enter the size, which should be the whole of the space. The area now becomes free space. Right click and select Create logical drive. You'll be prompted for size and which file system to format it with. Do those things and wait for the formatting to finish (i.e. until you see the word Healthy). Then you can exit Disk Management.