It is possible to get a list of all running processes using $com() and WMI (look at examples #2 and #3 in the help file regarding $com to get an idea). However, before I write such a script for you, you may want to look into an alternative: it is possible to make mirc run a program and
wait for it to finish, again using COM objects. This way you don't have to check repeatedly if the program's finished. All you need to do is install
this alias and use
/xrun -w winrar <arguments> instead of
/run winrar <arguments>.