one (or more) of your files ain't 'UTF-8 with BOM' (if your website won't be only English and use UTF-8 as encoding like mine). You must change all the files to 'UTF-8 with BOM'. Notepad++ will help.
there are some characters before php tag or after ?> (better take ?> out). Please be sure there are no any characters include space.
When call localhost/xxxxxx as the address, change it to 127.0.0.1/xxxxxx or your LAN IP
I always store sessions in DB instead of file (easier for debuging).