Warcraft 3 is my personal favourite and can be picked up for a mere tenner now on the bestsellers label, the expansion Frozen Throne is also available on the same label. I found this one quite easy to actually get into and if you fail a level you have the option to try again on an easier setting to get past it.
I loved Dungeon Keeper 2, one of my favourites to this day when it comes to RTS. However if you do get hold of it make sure it is fully patched to latest build up to be able to run on Windows XP without any probs.
Warhammer Dawn of War and Winter Assault addon is equally fantastic for a futuristic type RTS.
I have to admit I have a soft spot for Middle Earth so Battle for Middle Earth 2 was right up my street. Pretty easy to get into but I did find some of the missions to be quite tough. Also the game is buggy as hell and needs patching up to get it running right before you consider playing it.
A bit of a classic to finish off with is Shogun Total War (never played the expansion). Its pretty easy going and you don't actually have to fight all of the battles yourself if youre numbers are good enough. However the battles are very interesting if you have huge armies on each side, you can be going for a good while. Medieval Total War is just as good but I had probs getting it to run properly under XP. It just wouldn't save my settings at all so looked quite poor in the default low resolution.