How do I connect to a SQL Server 2008 instance on my network? I have 2 SQL instances in 2 virtual machines. I'm able to see the other instance in one machine but I cant connect to it.

Probably some permission problem...but I don't know where to look.

Can someone help me?

edit:
the first problem was in fact not having TCP/IP enabled, solved that and the error changed now I have error 18452. (I'm trying to connect with windows authentication, also tried with sql authentication in that case I get error 18456)

Both SQL instances are default

edit2:
After changing the autentication to windows and SQL, I can login in one of the instances using sql autentication (user SA), on the other...it gives me error 18470, but everything looks ok with that login (at least for me... maybe one of you can tell me were should I look)

edit3:
After enabling login sa everything works as expected in SQL authentication mode.

I change my question to, what if I wanted windows authentication mode? how could I do it?
To answer your question we really need more information like errors.

Firstly need to ensure your instance allows remote connections by configuring network protocols in SQL server configuration tool.

Also need to check if your connecting to a default instance or named.. 

If your still stuck post more information about your scenario.