I have used the following to determine the serial:

DECLARE INTEGER GetVolumeInformation IN WIN32API STRING @lpRootPathName, STRING @lpVolumeNameBuffer, ;
  INTEGER nVolumeNameSize, INTEGER @lpVolumeSerialNumber, INTEGER @lpMaximumComponentLength, ;
  INTEGER @lpFileSystemFlags, STRING @lpFileSystemNameBuffer, INTEGER nFileSystemNameSize
lcString          = ""
lcRoot            = "C:\"
lcVolumeName      = SPACE(256)
lnVolumeNameSize  = 256
lnVolumeSerial    = 0
lnMaxCompLength   = 256
lnFileSysFlag     = 0
lcFileSysNameBuf  = SPACE(256)
lnFileSysNameSize = 256
lnReturn = GetVolumeInformation(@lcRoot,@lcVolumeName,lnVolumeNameSize,@lnVolumeSerial, ;
				@lnMaxCompLength,@lnFileSysFlag,@lcFileSysNameBuf,lnFileSysNameSize)
IF lnVolumeSerial < 0
  lnVolumeSerial = 4294967296 + lnVolumeSerial
ENDIF

Not sure if this will work in Win98...