It looks that xar to xlsx conversion is typically searched by users that would like to recover Excel's auto backup files (.xar) and restore the standard spreadsheet from it (.xlsx, .xlmx etc.).
First and foremost, you have to be sure your .xar file is actually MS Excel auto backup. You can usually tell this by the fact, that these auto backup files have arbitrary filenames, such as ~ar15a.xar or something similar.
To recover these files, put them in "drive:\Documents and Settings\user_name\Application Data\Microsoft\Excel" folder and start Excel, it should automatically offer you recovery of your spreadsheet. Once this happens, you can freely save it to whatever spreadsheet format you need, including .xlsx files that have macro functions enabled.
If this does not work, then your .xar file is either corrupted or not proper Microsoft Excel auto-backup file.