In all four minidumps the WinDebugger symbols could not load to give code errors. Two of the drivers cited were core Windows drivers.
Another driver was nv4_disp.dll which is a Nvidia driver.
The last driver cited is yk51x86.sys which belongs to NDIS5.1 Miniport Driver for Marvell Yukon Gigabit Ethernet Controller and/or Adapter.
Do the following:
1. Run ChkDsk in the Run box.
2. Scan for viruses, etc.
3. Run MemTest for a minimum of 7 passes.
4. Run a full harddrive diagnostics.
* One other thing, did you have these issues prior to SP3?