Is it possible to install two versions of Microsoft office in one computer?
Yes, but you need to be careful which application you install first. There are specific rules related to the specific versions you use. Also, windows usually recognizes the last version as the default program for specific files. For example, if you install MS Office 2003 first, then install MS Office 2007, you might not be able to access the e-mail from Outlook 2003 without importing the e-mail files into Outlook 2007 (different file formats).
Unless you have a specific requirement for using two versions of MS Office on the same computer, it would be best to uninstall the first version before installing the new version.
Yes. I downloaded PowerPoint 2007 for use of a specific project, although I much prefer the 2003 version. I already had the 2003 version and was able to use both versions in tandem, and keep my 2003 as the default.