Computer system protection

Computer system protection to protect against harmful data from an external computer network 60 (e.g. the Internet) involves supplying incoming data 62 to a software checker 64 as the data enters a computer system (not shown). The checker 64 routes any suspect data 66 to an encryptor 68 which encrypts it to render it unusable and harmless. Encrypted data passes to a computer 72 in an internal network 74 and having a desktop quarantine area or sandbox 76 for suspect data. The computer 72 runs main desktop applications 78 receiving encrypted data 70 for storage and transfer, but not for use in any meaningful way because it is encrypted. Equally well applications 78 cannot be interfered with by encrypted data 70 because encryption makes this impossible. On entry into the sandbox 76, the encrypted data 70 is decrypted to usable form: it then becomes accessible by software 204 suitable for use in the sandbox 76 subject to sandbox constraints.