LxAdmin comes with Lighttpd preinstalled: it's a light web server that works fine in most cases.
Otherwise you can switch to Apache from inside lxadmin (remember, the switch works for all the domains hosted inside your vps, you can't have some domains with apache and some with lighttpd):
1. login as 'admin'
2. click on the 'server tab'
3. on the box 'server linux' click on 'switch program'
4. make your choice for the web server you prefer