This error will generally occur when trying to access a Server archive file while that archive file is being accessed by another process on the Perforce Server machine. Anti-Virus and Backup software are common culprits for this error.
The error is often caused by anti-virus software on the machine scanning the archive file, blocking the Perforce Server from accessing it. Configure the anti-virus software not to scan the P4ROOT directory and any of it's subdirectories. If the archives are mapped to a directory not under the P4ROOT, these will need to be excluded as well.
See Common Perforce Server Problems on Windows for more information.
Running a Backup whilst the Perforce Server is running can also produce this error. Running the backup at a quiet time or when the server is stopped will prevent this error from occurring.