Running illegal "cracked" software on a computer may lead to the loss or theft of personal data, security experts have warned.

Half of all cracker programs that claim to strip the copy protection from Microsoft's Windows Vista are actually malware-loading Trojan apps, according to John Safa, chief architect at security firm Drive Sentry. 

"Hackers are attaching malicious programs to original cracks and sending the mutated versions back into cyberspace," he said.

"Anyone who downloads and runs one of these infected cracks will also unlock the malicious program, which could cause irreversible damage to their PC or data."

Drive Sentry tested a number of Vista crack programs from file-sharing sites, and found that nearly half attempted to load malware onto users' systems.

While the problem is exacerbated by the malware authors and crackers, Safa put much of the blame on Microsoft and its decision to block developers and researchers from gaining access to Vista's kernel.

"By claiming that it has locked down Vista, Microsoft has effectively issued an open invitation to the hacking community to prove it wrong," he said.

The PatchGuard software used by Microsoft to seal off the Vista kernel has been criticised by security researchers, who claim that hackers will break the protections and have the ability to create malware that security vendors will not be able to stop.