theres no legal reason to stop you
The disk images are available on Digital River's site (M$ distributor)
http://msft.digitalrivercontent.net/win/X17-58996.iso Win7 Home Premium x86
http://msft.digitalrivercontent.net/win/X17-58997.iso Win7 Home Premium x64
http://msft.digitalrivercontent.net/win/X17-59183.iso Win 7 Pro x86
http://msft.digitalrivercontent.net/win/X17-59186.iso Win 7 Pro x64
http://msft.digitalrivercontent.net/win/X17-59463.iso Win7 Ultimate x86
http://msft.digitalrivercontent.net/win/X17-59465.iso Win7 Utimate x64
You can also link directly to another set from Dell's site (anyone can use them - Dell just posted for convenience)
Dell Community
For the sake of the mods:
These are not links to bootleg sites. These are links to official, legal microsoft downloads on microsoft sanctioned servers
Just remember that if you use these, you lose ALL software and drivers installed as part of the preload.
You should be able to activate them using the product key on the side of the machine. However in most cases you will have to activate by phone as online activation is blocked for most OEM product keys
Similar links for Office 2007/10/11 are available