Você pode não ter alterado todas as configurações necessárias.
De Como tornar as visualizações da barra de tarefas do Vista e as miniaturas de ALT-TAB maiores :
Save this to a .reg file and run as
Administrator:
Windows Registry Editor Version 5.00[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\AltTab]
"OverlayIconPx"=dword:00000023
"OverlayIconDXPx"=dword:00000000
"OverlayIconDYPx"=dword:00000000
"ThumbSpacingXPx"=dword:00000004
"ThumbSpacingYPx"=dword:00000004
"SideMarginPx"=dword:00000004
"BottomMarginPx"=dword:0000000a
"MinThumbSizePcent"=dword:00000064
"MinWidthPx"=dword:0000012c
"TopMarginPx"=dword:00000020
"MaxThumbSizePx"=dword:000000dc
"MaxIconSizePx"=dword:0000015e
"TextBottomPx"=dword:000001f4
I've also tightened up the borders and
margins a bit, to make room for larger
thumbnails. However, before importing
this into your registry you must edit
the MaxThumbSizePx to a size that
works on your monitor first! The
current value, above, is DC hex or 220
decimal, which works on my 1600 x 1200
primary monitor. The maximum row size
is 7, so each row is 7 x 220 = 1540,
right under the 1600 width.