The agreement will see CSC manage the retailers' portfolio and IT infrastructure, including servers, desktops, networks and peripherals.
CSC will also upgrade Allders' financial systems and implement a corporate intranet.
"We rely on tried and tested retail management systems, but had come to realise they weren't working as efficiently for us as we'd have liked," said John Tangaa, IT director for Allders Department Stores.
The retailer will implement CSC's e3 architecture, which Tangaa said would allow it "to combine our legacy systems with new applications - ensuring we remain in control of our technology at the same time as aligning our IT solutions to the company's expanding business plan."
Allders is following other major retailers, such as Sainsbury's, which outsourced its IT to Accenture in 2000.
CSC has recently won contracts with David Lloyd Leisure, Gallaher Group and Magnox Electric.
Buy.com to close
High street retailer John Lewis intends to close Buy.com in March. The company hopes most of Buy.com's 200,000 customers will move to johnlewis.com.
Murray Hennessy, managing director of John Lewis Direct, said it made little sense to run the two sites side by side. The company bought the UK operations of Buy.com last year in order to strengthen its Internet operation.