if the above doesnt work then you will have to try this...
you will need the recovery disk if you remembered to make 1.
if you didnt your stuck until you can find sum1 to lend you a disk with the same version of the o.s as you already have... just use its installer and enter your legit key.
Well for XP, I always just grabbed my OEM cd and used it(with the key from my COA), but I guess you need that disc.
Maybe this will help?
Detailed Operation Procedure
1. Repeatedly press and release the F2 key once turning on the computer until the Lenovo Rescue System open, then select OneKey Recovery.
Note: System Recovery will overwrite all of the data on the C: drive. To prevent loss of data, be sure to back up relevant data before performing system recovery.
2. Follow the on-screen instructions to select the backup task you want to restore from and the disk where you want to install the operating system, then press Next to start the restore.
3. Please wait during the process of system recovery. Do not interrupt the operation during the recovery process.
4. After the system is recovered successfully, the software will prompt you to restart the computer. Restart the computer and start the operating system.