|It's 2010, why are you still running SP1a?|
Dimension 4200? Are you sure it's not a 2400? All you need to do is go to the Dell website & download the appropriate device drivers:
Dimension 2400 drivers
BTW, here's where you can download SP3:
And one other suggestion...do NOT install IE8.
IE7 = yes, IE8 =no!