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:
Notice the opening double-quote. Then you will get:
cd "Program Files (x86)"/