I have a selection of photos on my PC that were taken in Landscape, but I want to make Portrait. The reason for this is so they can fit Portrait slots in a multi picture frame. I have chosen these pictures, as they have central focal points (such as a church), and nothing much at the edges of the pictures. The tools I have at my disposal are, Microsoft Office Picture manager 2010, and the photo printer at my local supermarket. I realise that I should only need to get the ratio correct, but is there an easy way to make sure my photos still look good once I have alter them? The pictures are of good quality and size (2048 v 1536), so the editing and stretching shouldn't mess up the quality when printed.
Your best bet is probably to crop the photos when printing, especially if you are unfamiliar with editing photos on your PC. Most of the photo terminals at supermarkets I've seen have had the ability to let you rotate and crop photos.
You will lose resolution when doing this, to avoid pixelation when printing you really need 300ppi (pixels per inch) for a reasonable viewing distance you can get away with 200 or 150 absolute minimum. At 200ppi you could print your images 10"x7.5" whereas cropped to portrait that goes down to 7.5"x5.75"
Assuming the images were sharp to begin with you should be able to get away with printing them 7x5.
The only thing you need to do to preserve quality when rotating and cropping is to save the images back using the highest possible JPEG quality setting (as JPEG is a "lossy" format you lose data every time you save) or use a lossless format like PNG, TIFF (some labs wont support these however). That's one advantage of doing the rotation/crop yourself, as you have full control unlike the machine at the lab.
You could probably get them cropped at one of the in-store developing terminals, but to be sure, you can crop the images in most image editing software -- it might be advisable to pull in a favour from any local friends that are into photography, but you should be able to do this yourself using Microsoft Picture Manager, which you mentioned you have available.
In Microsoft Picture Manager, open the picture, and from the "Edit Pictures" task pane, select "Crop" - the aspect ratio should match your output format - it has some common aspect ratios used by print labs (and therefore by frame makers) which can constrain the image to make sure you get the right size.
A simpler option would be to print them all so that they are big enough and trim them down with a craft knife and a straight edge.