I format and reinstall Windows Xp to a Xp pc. But after that I realise that some important files are in green color which means that they encrypted in formatted operating system. Now I cant open the files, i need to decrypt them but I don't know how to do that. Is is possible to get my files back?
See the Microsoft support article on decryption
Unless you saved the owner's public or private keys, it is possible that you will be unable to decrypt the files.
By far, the most frequent problem with EFS occurs when EFS encryption keys and/or recovery keys aren't archived. If keys aren't backed up, they cannot be replaced when lost. If keys cannot be used or replaced, data can be lost. If Windows is reinstalled (perhaps as the result of a disk crash) the keys are destroyed. If a user's profile is damaged, then keys are destroyed. In these, or in any other cases in which keys are damaged or lost and backup keys are unavailable, then encrypted files cannot be decrypted. The encryption keys are bound to the user account, and a new iteration of the operating system means new user accounts. A new user profile means new user keys. If keys are archived, or exported, they can be imported to a new account. If a revocation agent for the files exists, then that account can be used to recover the files. However, in many cases in which keys are destroyed, both user and revocation keys are absent and there is no backup, resulting in lost data.
Basically, you can't. Before doing the format/re-install, you should've decrypted the files. If you are in a domain environment, there is a recovery agent (thank goodness), so you can login as the recovery agent and decrypt the files