uTORRENT

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What are µTorrent's system requirements?

Very low. It works on as weak a system as Windows 95 on a 486 with 14MiB of RAM (with the Winsock2 update), up to 2003 and Vista. It also works on 64-bit Windows.

It's reported to work under Linux using Cedega with some issues (blue bars in Pieces and Files tab don't work, update function fails, minimize has a slight bug) if you use "Win98" mode.
It works very well under Wine 0.9.16 (especially with µTorrent 1.6), but versions previous to 0.9.15 might have issues or not work at all. There are some minor bugs.
It will also work under FreeBSD 6.1 using Wine 0.9.16+, but requires the GLX module to be enabled in xorg.conf.
It will NOT work in Darwine 0.9.12 on OSX. It is reported to work, however, in CrossOver Office alpha 2 and 3, though you can't minimize for more than a few seconds without it freezing.

All 9x systems should have at least IE 4.0 installed: it upgrades a system DLL that provides functionality for various µTorrent functions. However, it is not necessary for µTorrent to run. Without it, only the toolbar, graphs in the pieces view, and double clicks in the list view no longer function.

Be warned that Windows 95/98 are limited to 100 max concurrent connections, unless you apply the following registry tweak (sourced from Windows TCP/IP Registry Entries).
Open the registry editor (Start -> Run -> regedit), go to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP

You must create the following entry, it is not present by default.
MaxConnections
Data Type: String
Specifies the maximum number of concurrent connections. The default is 100. The maximum is 512.