If you don't mind following the trail set by a sixty eight year old whippersnapper, you'll have found out a few things by teatime.
Press your Windows key and R together and you see a small input box. In there, type the word
regedit
then hit the Enter key. You're now in the Registry which is Windows big filing cabinet and, as such, is crucial to the smooth running of the system. So, back it up before changing anything. You do this from the File menu (the Alt key and F will get you there or you can use the mouse) then select Export to make the backup.
Take a note of the name and location of the backup you're creating in case you need it later but it's pretty unlikely. The filename ends with .reg.
You navigate the Registry the same way you do in Windows file explorer so you click on H_KEY_LOCAL_MACHINE then on Software then Microsoft then Windows NT (noot just Windows) then on to CurrentVersion and finally, Profile List.
The entry you're looking starts witbh S-I-5-21 and ends with .BAK. First though, find the entry with the same name but without .BAK at the end. Right click it and select Rename then add .OLD to the end of the name. Go back to the file with the .BAK and double click it. Edit it by taking the .BAK off the end, leaving it with just the numeric bits.
Go back to the File menu and click on Exit. Restart the laptop and you should be able to get back in.
Pour yourself a drink of your choicce and congratulate yourself on saving the equivalent of £40 in British pounds and added something to the "can do" list.