If I remember right, the first sector is the mbr and boot code, and when the XT first came out, DOS was placed right after the MBR and that landed the first partition at sector 63.
I'm also remembering something about the first cylinder being reserved and there were 63 sectors per cylinder, so the first partition started at the second cylinder and thus sector 63.
Anybody know if I'm remembering correctly at all?
From an answer I've written on Super User:
This odd number is an artifact of cylinder-head-sector (CHS) addressing used in INT 13h, the legacy BIOS API used for disk access. On legacy systems and bootloaders that used the INT 13h API, all partitions must begin and end on cylinder boundaries. Even after logical block addressing (LBA) was introduced, fake CHS values (which did not correspond to the actual disk geometry) were used to maintain compatibility with the legacy API. Since CHS addressing originally supported a maximum of 63 sectors per cylinder, the first partition would begin on sector 63. Windows XP (before Service Pack 3) and earlier versions of Windows will not boot if the system volume isn't on a cylinder boundary.
In short, you did remember correctly.