"the trust relationship between this workstation and the primary domain failed" how to solve this error in windows 7? I couldn't even login locally?

If you cannot login locally, boot your machine with Hiren's Boot CD , use a tool called " Active Password Changer" , activate "Administrator" account and reset its password

now boot normally to your machine using Administrator account with blank password

Make your machine in a work-group, then join your domain again.

You can unplug the workstation from the network and then log in using the cache account with admin privileges. I assume that you have logged in an account with admin rights before while installing it with applications. And once you have logged in, insert the network back and rejoin it in the domain.

Hope this helps.

You can login with the existing user by unplug the network cable, then login using the this user. after that reconnect the network cable and rejoined the computer to domain. you might need to delete the computer name from your AD server if you will keep the same computer name.

You should be able to login if you know the correct user name and password. (If it's Windows7 you should put the computer name first and then the user name and password for login as locally. e:g - If your domain name is contoso.com, you should put contoso\\user name and correct password)

Always delete the old hostname in OU and then add it to domain after format.

Only option available now is reformat even if you use Hiren boot cd tools , the trust relationship error will be back again.

Remove your computer from domain with your Admin cred and re join with domain 

1: First You need to login locally as an "Administrator" , you can do so by removing Ethernet cable if you are facing some issue while logging in locally, then put your credentials.

2: Reset the Computer account through the Active Directory Users and Computers console.

3: Then rejoin the domain.

remove system entry from AD then loging workstation locally, after un joing  and rejoing to domain. your problem will solve. mostly this issue comes may be accidentally rested computer account from AD, Or Same Computer Name Used Another Workstation.

Login in to your Admin account. Then removing the system from the Domain and rejoining it would simply solve the issue.