Change directory with space followed by '('

You have to escape the parentheses:

cd Program\ Files\ \(x86\)

Pressing TAB will usually complete the command line for you, and will give hints how to quote the file/directory names.

Another way to accomplish the same is to quote the path containing whitespace:

cd "Program Files (x86)"

This also works with tab-completion so you can type:

cd "Program<Space><Tab><Space><Tab>

Notice the opening double-quote. Then you will get:

cd "Program Files (x86)"/