cd c: \folder1 (space only for anti emoticon purposes)
ren *.xls *.csv
Take care - that only works if the default user directory is on drive c:
If the default user directory is
d: \somewhere then
cd c: \folder1 will tell the OS to change it's logical pointer on
c: to
\folder1 but your
ren *.xls *.csv will try to take effect in
d: \somewhere
So you would actually need to include
c:
cd \folder1
ren *.xls *.csv
But given that the original
ren c: \folder1\*.xls *.csv was explicit it's better IMHO. With an explicit path in the command the default user directory is irrelevant.