About 80 percent of the department's computersbetween Monday, Nov. 22, and Friday, Nov. 26, as a result of the disruption.
EDS has completed its investigation into the cause of the crash and delivered its initial report to the Department of Work and Pensions. It states that an EDS operator error led to software upgrades being applied to computers that shouldn't have received them.
It is believed that the error occurred during the DWP's migration from Microsoft Windows 2000 to Windows XP.
EDS said the service disruption did not affect benefits systems or the payment of pensions and benefits but prevented 40,000 PCs from accessing those core systems.
A team of EDS and Microsoft engineers worked with DWP staff to resolve the issue, but it was not until Nov. 26 that a full service was available again.
EDS said several steps have already been taken to avoid this happening again, including increased checks by EDS' senior engineers and management staff when such upgrades are implemented.