Centrica, the parent company of British Gas, is migrating 26,500 users to Windows 7 from Windows XP.
The move will see Centrica's call centre and operational staff, through 26 locations across the UK, move over to the updated operating system, which has been available to businesses for around two years.
Centrica said the move would lead to improved operational performance and enhanced overall user experience. It would also enable the deployment of new applications in its call centres.
To manage the migration Centrica has extended its IT services deal with Fujitsu. Fujitsu already provided desktop managed services for Centrica's 37,000 staff as part of a five year deal agreed in December 2010. The "new agreement signals a deeper, more strategic partnership for the two organisations", said Centrica.
Worth a total of £4 million, the desktop transformation project kicked off in Spring 2013, and is set to run to the end of the year. Centrica will also adopt Windows 8 - designed for gadgets like tablets - among selected staff.
In April Africa's largest electricity company Eskom said it would be migrating 54,000 users to Windows 7 and Windows 8 from Windows XP.