1. Go to the following location:
usually C:\Program Files\Microsoft SQL Server\MSSQL10_50.QNEBSS\MSSQL\DATA
Confirm the .mdf and .ldf files are not compressed. If compressed, uncheck "Compress contents to save disk space" from the Advanced Attributes of the files Properties.
2. Grant the Windows user and the the "NETWORK SERVICE" account full permission to the before-mentioned Data folder.
3. It is also possible that the master.mdf file is corrupt. If steps 1 and 2 do not resolve, backup the current master.mdf and master.ldf (or mastlog.ldf) files, then replace them with known good files.