Curved Rainbow Text
This was a complicated affair, and probably done mostly because it looked like a challenge. I wanted to arrange some text on a curve, and I wanted the letters to seem to sit on that curve. Paint has a “Skew” option, but, as you've probably discovered, the skewing is only done to the right. It was easy enough to skew the v, e and s. Then, because the spacing between the letters r and v can be bad at the best of times, let alone when they're curved away from each other, I just had to skew the c, u and r to the left.
Here's the way to go about it.
Skewing to the Left
Select whatever it is you want to skew left.
Open the Image menu and choose Flip > Horizontal. 
Open the Image menu and choose Stretch and Skew
Type 10 into the place for horizontal skewing. (More than that distorts letters too much. You might want to try bigger numbers with some shapes or objects, though).
Open the Image menu and choose Flip > Horizontal
OK. It's weird—but it does work.
The next thing I wanted to do was arrange the text on a curve.While the text was still black, I drew a red ellipse below it. I selected one letter at a time, skewed it, then dragged it down and sat it on the red ellipse. When all of the letters had been arranged, I picked up the flood tool (I still had red on my left mouse button) and flooded the whole page with red, followed immediately by white. I then selected the text and inverted the colours.
Rearranging and Treating Letters Individually
If you know that you're going to be pulling a word to pieces, don't type the word normally. Type letter, space, letter, space. That means that you won't have to mess around with the irregular selection tool. There'll be enough room between letters for you to select with the rectangular one.
The Rainbow Background
The picture of the finished rainbow coloured word is here.
Painting the rainbow background was easy, but if you take some time you'll be able to do a much better job. I clicked the line tool, chose the thickest line, then selected the ellipse tool. I drew thick ellipses of different colours, one inside the other, with big gaps between them. Then I flooded each gap with one of the adjacent colours.
Cleaning up the Jaggy Edges
I dragged the mask onto the rainbow background and removed everything but the text. Yuck! It looked awful. Very, very jaggy. Paint had “anti-aliased” the edges, but that had just made things worse. There was nothing for it but to zoom in and tidy those edges. I set the line tool back to a one pixel width and put black on my left mouse button. At first I drew lines and curved lines, but I had to finish by using the dreaded pencil to tidy the edges properly.
I replaced edge-smoothing greyish pixels with black where I thought the edge should be and the rest with white or the colour nearest to them.
If you have the patience, you can always trace the edge of each letter in a heading with a contrasting colour. I'd only do it when there was no other way out, though, because it takes time and can make your mouse hand pretty sore.