First get some windows working so you can check whether it has SLIC 2.1 activation. I recommend you install from OEM media the version you intend to use. (for example Windows 7 Pro). If you don't have OEM Pro CD, you can find online, you can also change the language pack after installation.
If you don't have a DVD burner or don't want to pollute environment by wasting a DVD, you can install from USB. You don't need any Rufus or MS utility. You just format USB in NTFS, make partition active using diskpart in administrative CMD and copy contents of .iso (extract with 7-zip) to USB key.
You can check that with Aida64 (download BETA) - you don't need to activate windows yet.
If it has SLIC 2.1 or higher then proceed with steps below. If SLIC is only 2.0 or lower then it supports only Vista. Sometimes on OEM machines a BIOS update adds SLIC 2.1. Go to Lenovo site and update BIOS and all firmware - you don't need activated Windows for that. If the sticker is Windows 7 it's most likely 2.1
Then you need to find the certificate in the form of xrm-ms and OEM key that matches it.
Make a folder usually I create folder called oem in C:\Windows\System32\
Copy xrm-ms file there (you need to confirm to make changes...)
Start command window in administrative mode, (right click CMD icon, > Run As Administrator)
Type:
slmgr -ilc C:\Windows\System32\OEM\certificate_file.xrm-ms
slmgrp -ipk XXXX-XXXX-XXXX - put your Lenovo key - you need a Lenovo key for your version of Windows 7 (Home, Pro, Ultimate..)
Reboot, in Computer > Properties it should show as activated now.
If after BIOS update it still shows as SLIC 2.0 you may find modded BIOS on my digital life or there is some Chinese forum which is difficult to navigate and you need to register to download. If you don't manage to have BIOS to have SLIC 2.1 (no OEM or modded BIOS available), then you need to call Microsoft and activate by phone, you need a valid key and they usually don't complicate if you say you reinstalled.
Activating using SLIC 2.1 OEM certificate and OEM key is much better than calling as in case you change too much hardware it won't activate and you'll have to call again. With OEM SLIC activation you can stick HDD in another OEM machine and it will work which is very convenient for places with multiple PCs.
You can get the certificate and the key from a working OEM machine, such as Thinkpad or find one online. My Digital Life forums are a good place to start.
You can also find Windows 7 48 in 1 or such DVD images where you can select install Lenovo Pro OEM. You can go this route if you trust the maker of the image (usually popular images are OK).
You can extract key from existing install using magic jelly beans keyfiner (google) but be careful to unselect crapware it comes with.
After that you install Lenovo System update or just install drivers for missing stuff in device manager from Lenovo site. Don't install any not needed bloatware such as Lenovo
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If you really need help, you can PM me.