To prefix a set of lines I use one of two different approaches:
One approach is the block select (mentioned by sth). In general, you can select a rectangular region with ctrl-V followed by cursor-movement. Once you've highlighted a rectangle, pressing shift-I will insert characters on the left side of the rectangle, or shift-A will append them on the right side of the rectangle. So you can use this technique to make a rectangle that includes the left-most column of the lines you want to prefix, hit shift-I, type the prefix, and then hit escape.
The other approach is to use a substitution (as mentioned by Brian Agnew). Brian's substitution will affect the entire file (the % in the command means "all lines"). To affect just a few lines the easiest approach is to hit shift-V (which enables visual-line mode) while on the first/last line, and then move to the last/first line. Then type:
:s/^/YOUR PREFIX/
The ^ is a regex (in this case, the beginning of the line). By typing this in visual line mode you'll see '<,'> inserted before the s automatically. This means the range of the substitution will be the visual selection.
Extra tip: if your prefix contains slashes, you can either escape them with backslash, or you can use a different punctuation character as the separator in the command. For example, to add C++ line comments, I usually write:
:s:^:// :
For adding a suffix the substitution approach is generally easier unless all of your lines are exactly the same length. Just use $ for the pattern instead of ^ and your string will be appended instead of pre-pended.
If you want to add a prefix and a suffix simultaneously, you can do something like this:
:s/.*/PREFIX & SUFFIX/
The .* matches the whole line. The & in the replacement puts the matched text (the whole line) back, but now it'll have your prefix and suffix added.
BTW: when commenting out code you'll probably want to uncomment it later. You can use visual-block (ctrl-V) to select the slashes and then hit d to delete them, or you can use a substitution (probably with a visual line selection, made with shift-V) to remove the leading slashes like this:
:s:// ::