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Doing a clean reinstall of Windows is dead easy now, you just download the Fresh Start tool and it does everything in a couple of clicks. The Windows license is tied to the motherboard, so you don't need a CD key. If you install Teamviewer, you can remotely connect to her computer when she inevitably needs tech support.
The tobacco is a harder problem - the smell of fags deeply permeates your stuff, but you can't really notice it yourself. If you take the machine apart, you'll probably find a gunky mixture of dust, fluff and tar covering the fan and heatsink. Personally, I'd tear down the machine and wipe down everything I can see with plenty of isopropanol; if you can't find isopropanol at a reasonable price, get a bottle of bio-ethanol from B&Q.