Windows 7/Vista Supercomputer?

OK, now I know there are some older threads regarding this issue.
First of all let me present you the situation.
At school we have a laboratory with Celeron-Based computers (Celeron from the core 2 duo generation not i3's etc.) with 2GB ram, with Windows Vista Business in the near future with Windows 7 PRO.
We have different situation when we simply cannot do what we want with them, eg. Render a Movie, working with a Mysql-Site setup, Joomla! to be more precise, the computers is at 100 % procent and I am always scared that he could restart and ruin my work, also we have a server same celeron-Based, I think it's top celeron model and 4GB ram, now primary we use a MYSQL-Joomla! workstation, and some C++ programing, but the c++ it's not an issue, from time to time we render some movies like a presentation.
I've read some cloud computing articles and I hope someone know a way to increase a single computer processing power using other idle or with low workload computers inside the LAN.
Also I've read something about Dropbox, but seems only o save files on all computers that's all.

Any help will be appreciated.

~Darkeloth