With older CPUs the HAL(Hardware Abstraction Layer) cannot detect the correct amount of 2nd level cache, which is the case with Pentium 1 using direct-mapped L2 cache (And sets it to default 256 KB).

To force usage of a certain amount of 2nd level cache in KBytes, edit this DWORD registry key:

[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Control \Session Manager \Memory Management]
SecondLevelDataCache=512 (Default AutoDetect - 0)

Note newer processors like the AMD K6-2 and Pentium2 are using set-associative 2nd Level cache and are detected properly. Force setting the second level cache for such CPUs (And newer) will probably do more harm than good.

More Info MS KB183063
More Info MS KB228766

Credits arstechnica.com