Over time, I've discovered that a few of the icons from Windows XP actually exist in sizes larger than their standard 48x48 size. So here they are, in wonderful 256x256 resolution: Download High-Resolution Windows XP Icons
So where exactly did I get these? Well a couple of them came from various Longhorn builds, but most of them were put together from shapes from the Windows XP tour. Some of them I had to kind of put together myself (for example, the 'My Network Places' won't be seen in the tour, but what can be found are globe and computer monitor icons, which can be put together to create the 'My Network Places' icon). Also, the 'My Profile Folder' icon is custom-made for use on Windows Vista and newer for the username folder on the Desktop. If I find any more, I'll go ahead and update here.
Do you have a list of the Longhorn builds you extracted some of the other icons from? It would be cool to assemble a full collection of icons (such as what is shown in shell32.dll) entirely vector to make a scaling XP Linux theme with.
Unfortunately, only a few of the icons from Longhorn are high resolution (all of which are already included in the download here) and none of them are vector. The vector ones were extracted from the Windows XP tour, which I converted to SVG and then ICO. If I had a full collection of high-res/vector XP icons, I would have posted it here already.
If I come across any more, I'll update here though.