Execution time of adding an index is too long

Hi all,

I have a table of 45 millions records.
I am trying to create an index on a column.
I run this job on Friday, today is Monday and this job is still working. Is there any chance to improve this situation?

mysql> show processlist;
±----±-----±----------±-------±--------±-------±----- -------------±--------------------------------------------- ---------+
| Id | User | Host | db | Command | Time | State | Info |
±----±-----±----------±-------±--------±-------±----- -------------±--------------------------------------------- ---------+
| 823 | root | localhost | my_db | Query | 260699 | copy to tmp table | ALTER TABLE my_table ADD INDEX datetime (datetime)
±----±-----±----------±-------±--------±-------±----- -------------±--------------------------------------------- ---------+
Coping into the “tmp table” is normal process or I can improve it by somehow?

P.S. MySQL 4.0.45 / engine type: MyISAM

Thanks in advance.