Built my first computer

I use a DELL 4100 case & motherboard. I reformatted a 20GB hard drive,installed XP Home, 2ram sticks =384MB,replaced powersupply & modem, all from other computers. My problem is its slow to boot,I’ve lost the partitions,and it keeps telling me install diskete in (A)drive. I dont hav much money so i built it outa leftovers.
Any help would be great. Pappy