This weekend (21 November 2009 & 22 November 2009) there is no Central Line at all west of Marble Arch. That is a huge chunk of the Central Line that will not be working due to engineering work.
If you are traveling out from Central London heading to the West Ruislip branch this weekend, the advice is to travel to Willesden Junction on the Bakerloo Line. You will then be able to pick up one of a couple of rail replacement bus services.
If you are heading for Ealing Broadway, there is a replacement bus that starts at Sheperds Bush Market (Hammersmith & City Line), which also picks up at North Acton (where the West Ruislip branch buses will be able to drop you off to change buses). I think I'd rather travel out on the District Line or catch a First Great Western train from Paddington to Ealing Broadway.
Chiltern Railways also offer a regular but infrequent service to West Ruislip and South Ruslip from London Marylebone. There is also a half hourly service for most of Saturday from Paddington to Greenford. Do check National Rail times before you travel on a big train.
There are loads of alternative train, tube and bus routes - I can't possibly summarise them all, but it could be a good opportunity to check the accuracy of the tfl Journey Planner. I've just run a sample journey through it (work to where I used to live, on Saturday, and it was spot on for that sample).
I am glad that I chose to go to the Westfield Shopping Centre last Sunday and not this coming weekend.