We have OLTP Databases on SQL Server 2000 on my client's Prod Server. Planning to create Data Warehouse on the same Server but use SQL Server 2012. Can I install without any issues?
As stated on MSDN side-by-side installation of SQL 2012 with earlier version is supported. However take note of the instance name, if your SQL 2000 instance is a default instance then your SQL 2012 install will have to be a named instance.
I would take note also that some issues may occur with SQL 2000 tools as your SQL Server 2012 installation may attempt to upgrade any shared components it finds. Although I don't know what of SQL 2000 would still exist in SQL 2012.
Now, my opinion and not the opinion of StackExchange, or my dog...I would not suggest configuring a side-by-side install with a SQL 2000 instance. Not knowing the setup, I would highly doubt the server configuration of a SQL 2000 instance is very well suited to what Microsoft suggest for a SQL Server 2012 installation. Especially considering the fact that SQL Server 2012 is only supported on Window Server 2008 SP2 and higher. This all depends on how old the server is and such but just my opinion.
You should be able to run separate instances of those versions on the same server without issue, though I usually recommend against doing that in production environments as I have experienced problems in our dev environments with updates sometimes refusing to apply (I forget which, but for us one of the SQL2005 service packs refused to install on the machines that had both SQL2005 and SQL2008 instances present).
SQL2000 does not reach EOL until later this year but it is in the "extended" support period, see http://support.microsoft.com/lifecycle/?LN=en-us&x=14&y=10&p1=2852, so it is likely that MS have tested the combination as part of their release testing process but if you do have problems that are not security related you'll end up paying for resolution if you need to refer to MS support. I would therefore recommend fairly rigorous testing of your own before you try this combination in a production environment. Also see Shawn Melton's points regarding SQL2012's system requirements, which may mean that your old SQL2000 box is not suitable even though it is theoretically possible.
I tried to install SQL Server 2012 and 2014 on the WIN2008R2 server with Sql Server 2000 Desktop Edition named instance installed. The installation process failed showing error:
TITLE: SQL Server Setup failure. SQL Server Setup has encountered the following error: '.', hexadecimal value 0x00, is an invalid character. Line 1, position 444345. Error code 0x84B10001.
After taking a brief look to the log files I concluded that there may be some unexpected values in the registry with SQL2000 that cannot be processed by SQL2012/14 installation process.
Sure you can. Just note that you have to install an older version first, and then install the newer one
I have not tried your exact case, but you will certainly experience some problems (deprecated functions, sql instructions...). You will also have to name your instances carefully.
I would much prefer having multiple instances with the same version. If you wish to migrate your 2000 database, keep in mind that according to the MSDN, the migration from Sql Server 2000 to Sql Server 2012 is not supported, as you need an intermediate step, usually Sql Server 2008R2.
[25/03/2013 Edit] my initial message looked a bit off-topic.
Yes, you can install SQL 2000 and SQL 2012 as separate instances on the same machine without issues.