Can you give some idea of the price that shes prepared to pay Clive ?
If I was building one now I would go for one of the new Intel Core 2 Duo CPUs.
The best performance/value for money one seems to be the one that runs at 2.4ghz and has 4mb cache on each core, I cant think of the model number at the moment, theyre selling for around £190 inc vat at the moment if thats not out of her price range.
A suitable motherboard would be anywhere between £50 and £150.
I would suggest 1 gig of DDR2 ram minimum, preferably 2 x 1 gig running dual channel, decent ram should be just over £100 for 2 x 1 gig.
You will probably be wanting a PCI-e graphics card, as you havent mentioned gaming then one of the 6 series Nvidia ones should be ok starting at around £35 up to the new 8 series starting at just under £200 and going up to almost £400
Then of course you need a case and a PSU, even for a modest pc I would now be going for at least 400 probably a 500 watt PSU, again around £30 should get a decent enough one, make sure that it has the P4 connector and a PCI-e connector as some still are being sold without them.
A hard drive and DVD writer/s are the only other thing that she will really need.
Big hard drives are relatively cheap these days and shed probably be ok with 160 gig upwards but I tend to be using 250 gig or bigger these days as the price per gig is cheaper the bigger you go and rather worryingly I am filling them up quite easily, I have 3 x 250 and 1 x 160 drives in my main pc
A 160 gig drive should be from around £60 and fast, dual layer DVD writers are starting at around £30.
Cases again can cost from £20 and go up to a couple of hundred.
So you are looking at around £450 for a good basic pc that will be Vista ready and capable of all she will need for many years to come and another couple of hundred if you want a good case and more ram and hard drives.
If thats a bit much then she can get a very good Intel dual core 945 CPU for under £100 which is about the best value CPU in the up to £100 price range from Intel, some AMD CPUs may be cheaper but if shes not gaming and wants reliability then Intel is the best to go for.