In order to put launch conditions on your install, you will need to open your project's .ism file in DevStudio. Here, under the Installation Designer, go to the General Information > Product Properties view in the IDE. From here you can edit the Install Condition property to put installation conditions on your setup.
If you want to check the OS, you can use the VersionNT and Version9x properties.
If you want to check the version of Explorer, you can use the System Search wizard to search the registry for the registry key
HKLM\Software\Microsoft\Internet Explorer and the Registry value of 'Version'. You can then use the property the system search sets as a condition.
For disk space, you will need to write a custom action to handle this, as there is nothing built into MSI that can handle this.