To avoid accessing the HDD every time to read a file, it caches the most used files or at least the most used parts of the file. The RAM allocated for disk cache cannot be used by applications, so if having a limited amount of RAM one should consider the amount of memory used for disk caching.

Edit the file C:\Windows\System.ini and find the section [vcache] and write the following:

[vcache]
MaxFileCache = 8192
MinFileCache = 1024
ChunkSize = 512

The above values means that it will take 8192 KByte of your memory to use for the virtual disk cache. The memory will be allocated in chunks of 512 KByte.

One can use the following table to find a suitable value:

Installed RAM	MaxFileCache	MinFileCache	ChunkSize
32 Mbyte	2048	1024	512
64 Mbyte	4096	1024	512
128 Mbyte	8192	1024	512
256+ Mbyte	32768	4096	1024

More Info MS KB108079
More Info MS KB225497