My interpretation of your question is slightly different to that of user unknown. However, I agree with him/her that more information would have been a help. How many rows and how many columns? Can you show us a typical row?
I interpret "... displayed as plain text organized in columns and rows" as meaning your import data has fixed width columns. Excel is pretty good as guessing the boundaries between fixed width columns and you can adjust the boundaries easily if is guesses wrong.
"A friend of mine told me to put it into notepad and do something to it (like turn it into a comma-delimited format) but he did not finish explaining." Thankful I have no friends like this. Turning fixed-width-column format into comma-delimited format would take for ever with notepad. If this file is as big as you suggest, come back in five years with your CSV file and we will try to help you with the next step.
Excel 2010 will accept much larger files that Excel 2003 but may still have its limitations. If your file is too big for Excel 2010, Notepad++ would allow you to chop into ten parts, say, relatively quickly and with little fear that you had accidentally deleted something.