How are you getting the sound from your boombox into the computer? Does your sound card have a "stereo line-in" on a 1/8" mini jack which you are getting from the "line out" of you boombox"?
What kind of "file" are you creating in your computer from your boombox, ie MP3, WAV ?
If you are doing this as a true 16-bit CD quality WAV file you can't record the entire tape at once. The file size is too big and you have probably run out of RAM. Each minute of CD quality WAV (16 bit) file is 11 megs per minute. A 4 minute song is 40+megs of file that must be saved to your hard drive, named so you can use it later to create your play lists (drop and drag) you will then burn CD's from. You should do this a track(song) at a time.
Each side of your cassette tape is probably close to 30 minutes in length and would equate to over 300 megs of ram storage which most people don't usually have. Remember there are programs running that are sitting in your ram while you are running this program, also using your ram memory.
Making CD's is a little more complicated than most people think. Most consumers are better off buying a stand alone CD recorder from either Sony, Teac, or others.
www.musiciansfriend.com
Sony has one for about $279.00 that works just like a standard tape recorder and you transfer tapes in real time like you are used to doing using standard RCA type cables from your real stereo.
Hope this helps.