If you are familar with how Windows works, it can probably be done. The trick will likely be that before attempting to install, you will need to first remove references to version 10 from the system PATH and be sure that ORACLE_HOME is not set. Next you will need to understand that you won't be able to use both at the same time. in fact, it is unlikely that you will be able to have enough RAM installed on an XP machine to support Forms/Reports 11. The minimum requirement for the install is 2gig RAM installed and I believe 4 available (this means at least 2gig swap). That is the minimum. I can tell you from experience that if you don't have at least 3gig of RAM installed you will hardly be able to use it. Also, be sure that you read the installation guide and system requirements guide. You will need to have a static IP address or install the Winodws Lookback adapter before attempting the WLS or FMw installation.
So, the short answer... yes, it can be done, but I would not recommend it. ;)
1. In the windows's Path variable keep Forms10g related path values first and then Forms11g related path values.
2. Uncomment and add Forms11g related path values to PATCH_PATH variable from DOMAIN_HOME\bin\setDomainEnv.cmd file.
With this Forms11g will find its files in the PATH