Since it just started last night why not try System Restore?
Computer is an 1.6 AMD Athlon XP 1900+, MSI MS-6378 Motherboard, 768MB SDRam Running XPSP2 Pro. Only new Addition is An Micro Advantage DVD+R/DVD-R /CD-RW Superdrive and it's Easy CD & DVD Creator 6 & PowerDVD software. All Drivers and sotware are current with the latest updates. Now the Problem. Last night all The Partition letters in Disk Management, including C Disappeared. The Computer still seems to be running fine. All you can do with these Partitions in disk management is Delete them and make new but, still no drive letters.Partitions can be accessed through My Computer, but I still need to fix this. No matter what I have tried it doesn't work. Help!! Any assistance will be greatly appreciated.