It should be noted that 10Mbit/s is a puny transfer rate for a PC.
A tiny Mini-ITX (for example the quite inexpensive Intel boards with an Intel Atom processor) can handle many times that speed and write down to disk.

The only problem is that all PC OS requires that you somewhere below the OS (or on your transfer link) has a buffer that can support maybe 0.2-0.3s of buffering. Ethernet isn't normally a problem when it comes to latencies in the kernel. An IDE optical disk is normally the worst source.

Note that if you don't require this to be a standard Windows installation but are allowed to play with the OS installation, then you may consider the Real-time Linux modification of the Linux kernel. Then you should be able to capture your data to a buffer, and then let the Linux kernel take care of the data and write it to disk or send it to a user-space application for analysis. But the requirement is that you find a good PCI or PCI-E board that supports synchronous transfers at these speeds.

If this is a once-off, then you may also consider looking for a ARM9 development board with a 100Mbit ethernet or USB2 and enough RAM to buffer maybe 0.5 to 1 seconds and then use the prototyping area to add the synchronous interface - unless the ARM9 is used as the data collector.