Needing more than task managing
There is this tool that I have been using for quite some time now called Process Explorer. Basically it is an enhanced version of Task Manager and its open source.
It shows all the processes running on a machine just like you can also see in Task Manager but what I find very useful with this tool is that it shows the company name as well as the user name plus other options that you can add on depending on what you are trying to investigate. When I look in Task Manager I can see the process names but more often than not I cannot tell much from some of the names and this is where I like the Process Explorer. It shows the company name for example and if the name doesn't sound familiar then its probably a trojan and then you can then try to find out more info about that process.
You can also select between handle and DLL mode if you would like to see for example processes that certain handles or DLLs have opened or you can utilise the search facility.
Information about CPU usage and memory can be found through the System Information window and again you get more information than you would get through Task Manager's Performance tab.