I think I mentioned this somewhere, but oh well.
XP SP2. 500MBytes after housing .NET Frameworks and DirectX updates. Otherwise 300MBytes initially.
If you have time, spend some hours on vLite and nLite. It makes a difference.
I got a 2GB Vista install before.
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cChip interpreter WIP - current status: Release Candidate
LRx Filter RC - current performance rating: 9/10