I have a big problem I work under Windows XP but I still use some old DOS programs. But one of them need to load data in upper memory and use a DOS command named LH (loadhigh) to do this. But, under WinXP, this command doesn't exist anymore. Is there a way to use my program under XP (using LH or an other command) or must I come back to an older OS ? If I've got to, can I easily install Win98 and uninstall it, without uninstalling XP ?

The newer Windows gets, the further it gets from DOS. If you are going to need to use this command often, but want to keep XP, you can always go dual boot (meaning you have both versions installed.) Best way to do it is to have two hard drives or partition your existing drive into two drives. Format the drive(s) and install Windows '98 first on the first hard drive/partition. Then install Windows XP on the second hard drive/partition, when it asks, say "Leave current file system intact". Then every time you boot, it'll ask Windows 98 or XP.