hard drive

hi i’m joe
i upgraded to win98se from WinXPpro which was freezing up all the time.
i had a 20 gb hard drive to start, it was
partitioned into 4 drives C,D,E,F.
now win 98 only sees 15 gb and 3 partitions
C,D, and E. where is F? can i bring it back by reinstalling win XP??
please help thanks
joe