According to the datasheet, you have a 64bit-Sempron.
Can you reinstall the CPU-driver please?:
http://support.fujitsu-siemens.com/Download/ShowDescription.asp?SoftwareGUID=1A11D522-CDFE-44E4-9595-024BF70C1295&OSID=665F4A20-6E31-43C3-82C2-D98CE773007C&Status=True&Component=AMD%20Sempron%20Mobile%2062W
Since you used the DirectX Diagnostic Tool, which is a Microsoft-product, it uses the Windows-Interfaces to get the displayed information. So it won't tell you something else about the CPU-frequency than any other Windows-panel.
Please use a third-party-tool!
A (very) slow response is normal on a singlecore-computer, when something like video-encoding s done.
WinXP64 has a poor driver support compared to the 32bit-version. Maybe your problems with the installation were not caused by a missing 64bit-architecture.
Once again please:
Which encoder (software, not codec) do you use to encode your videos? And how is the RAM-load?