For quick and dirt stuff I like to use LView. Do a right click and save as:
There is no install, you need to make a shortcut to the exe. It is a very small put powerful program, just go to edit and resize. It does all sorts of other things too. Screen caps and such. It's pretty easy to use.
Open the file you want to resize with LView: File > open
Use the edit > resize (not redimension)
Select size (ie 800 X 600)
Drop down File > Save-As
In the bottom drop-down select .jpg
For the "file name" you must name it with the name you want and a ".jpg" extension, it won't put the extension on for you.
another thing with this program, you can open any type of graphic (almost) and do a save as in another format eg tif bmp jpg etc. You just need to remember to select the type in the drop down and in the "File Name" you use the proper extension.
If your computer has the generic Microsoft Photo Editor or other editing program, you should be able to select the option 'SAVE AS' at FILE and then click on 'Safe as Type' (jpeg, bmp, gif, ect) after you'd done with editing/resizing.