A search on Google for "ubuntu 6 ibm thinkpad t41" brings up a scattering of quotes suggesting people have encountered some difficulties working with Ubuntu 6.x on IBM Thinkpads.
There isn't an easy way to revert back to your pre-Ubuntu 6.x installation, since GRUB is now your system's bootloader. Since Windows works, I don't recommend attempting to remove the GRUB installation.
If you format the new partitions you created for Ubuntu, GRUB will still ask you if you want to boot Ubuntu or Windows. In that case, you'll just always select Windows.
My advice is to get a copy of Ubuntu "Breezy Badger" (Version 5.10) and try installing that into the partition where you currently have Ubuntu 6.x. The installation should see your GRUB installation and it should add options to boot Ubuntu 5 to your current operating systems list. The install probably won't remove the Ubuntu 6.x systems from your boot list, but this can be done manually by editing (carefully) the GRUB menu.lst file.