This is how I see the money for a backup and restoration of a windows operating system if being done on a mobile basis

Assume your travelling for 20 minutes to this customer, if you take into consideration costs associated with running a vehicle (fuel, tax, insurance, tyres, servicing) and the travelling time back home of 20 minutes you must expect to obtain at least an hours money unless your time is free ( £10 )
You arrive at the door to do the backup and the customer states that they can not afford to lose pictures of the holiday, music on the hard drive or the emails, but the system does not boot at all... So you take 10 minutes either removing the hard disk to setup as a slave drive in a working Windows based PC or you run a Linux live CD and read the drive in that manner... however finding all this information takes 10 minutes and you have to write it to something... are you going to provide free CDR's and DVDR's - the amount of time and knowledge to recover the data, burn it to disk, scan it for viruses must be worth ( £10 )
You then set out to format the system only to find that they have no Windows CD...what do you do charge them for a new one, or lend them yours and be called back later when they go to windows update and find out that the 'version of windows they have is not genuine'
But for a moment lets assume they buy a new Windows disk, do you think they have the driver disk that came with the printer, scanner, the Microsoft Office disk that was borrowed from the office, all the applications they had installed and want back!!

Installing Windows from scratch and playing hunt the drivers, hunt the applications and then installing them and setting up all the hardware such as scanners, printers, digital cameras, webcam etc, reconfiguring email must take an hour if all goes well ( £10 )

You will always end up doing more than you expected but this is typical of the person you will encounter... you have simply spent two hours for £30 and if you are a legitimate business by the time you have paid tax etc you have very little... if the customer has a problem and you need to return to them you practically end up with nothing for your time.

its a mugs game imho