I have a Dell Dimension E521,which has recently developed a boot problem.The BIOS half loads,and then sticks for an age,maybe 20/30 mins before going on to complete the boot/load.I have recently upgraded from Windows Vista 32 bit to Windows 7 32 bit,but this did not cause any problem initially.

Hello jnickcrispin
Go back to a previous date when your computer was booting smoothly . Use System restore for this : go to start , type restore in the search bar and select system restore .
Maybe there is a malware infection somewhere in your system : download and run MalwareBytes from
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html and scan your pc .
Once done , create an image of your entire system , it's easy under Windows 7 : go to start type backup
in the seach bar and select back up entire system . At the end of the operation , click Yes to create a System Repair Disc . Do this regularly , every time you made significient changes or saved new important data on your machine .
There is of course another option easy and radical if you own the Win 7 install disc : install a new fresh version of Win 7 but save all needed data before proceeding !
Best regards ,
fanaudi .

This issue could be the hard drive being slow to spin up, it could also be a bios or driver issue. Try flashing the bios to the most current version and see if there is a new sata controller driver on the Dell website.
You might try a different hard drive and see if a different drive has the same issue