I think your computer is probably fine. Might be a *little* sluggish feeling, with a 1GHz Celeron, but it shouldn't be so slow that the clock doesn't update properly.
640MB seems like plenty of memory, too. My development PC only has 512. However, what we've seen with a few users is that memory can really be key. *Having* 640MB in your PC makes no difference if there's little *free* memory.
Bring up the Task Manager and see how much memory is available. Press Ctrl+Alt+Del, or right-click on the taskbar and select Task Manager, to open the Windows Task Manager. Select the Performance tab. On the bottom of the dialog, see what these amounts are (when the TD is running):
Available, under "Physical Memory (K)"
Total, under "Commit Charge (K)"
You can copy/paste these amounts, although you might need to pause the display to do so, since it will keep updating. To pause it, on the View tab, select Update Speed -> Paused.