You cannot "upgrade" from XP to Windows 7, you must do a complete new (clean) install, either to a separate partition on the hard drive or to a new or reformatted hard drive (the preferred option). If this is not what you did, that is probably the problem. You will also need Windows 7 (or at a minimum Vista) drivers as the Windows XP drivers are not compatible in most cases (especially the audio). Also, on a new install (or reinstall) the correct and required install sequence is (1) Install Windows (2) Install chipset drivers (3) Install device drivers including the Dell audio drivers (for integrated audio), video, ethernet, etc.
Dimension E510 with D925 CPU and Vista Home Premium 32 bit
Home Built PC with Q6600 CPU, 8GB RAM, HD4850 Video, HTO Striker 7.1 Audio Card - Windows 7 64 bit Professional
Sonar 8.5.1 ( Producer) DAW Software/Focusrite Saffire Pro 40 Recording Interface and Frontier Tranzport wireless controller.