Thank you for your replies.
I tried FreeDOS, doesn't start.
I founded MS-DOS 6.22 bootdisk from: http://oldfiles.org.uk/powerload/bootdisk.htm
Booted it, fdisked and formatted drive, now it has operating system on it and it works!
BTW you can get more than 640 kilos of memory in MS-DOS with HIMEM.SYS and EMM386.EXE (XMS or EMS, not conventional).
Off topic: I bought this "laptop" today from the local fleemarket. Previous owner had used doublespace to double 40 meg HD to 150 megs!!!
When I scandisked it there was over 100 bad sectors... didn't finish that scan, but after formatting it back to 40 it had none of the bad sectors. Tell me, is this one of the worst DOS misuses ever or what?
EDIT: T5100 doesn't have CD-drive... yet...