VirtualPC, being a Microsoft product, does not support anything but the Windows platform. Only Windows guests and Windows NT-based hosts are supported. VirtualPC is a lot more immature than the competitors because it's a quick and dirty solution. The BIOS probably has some annoying bugs in it. Try searching for Solaris x86 disabling ACPI. (It'll be an option that you pass to the GRUB boot loader that first appears)
Since VPC isn't very portable if you want to move the images to another operating system, and since it really isn't a proven solution, I'll recommend VMware Workstation, which directly supports Solaris, FreeBSD, Linux, Windows, and OS/2. Alternatively, there is Parallels Desktop (Mac OS X), Parallels Workstation (Windows/Linux), QEMU, and VMware Server (As it sounds, leans toward servers, but is free, while workstation is not)
There is advanced features in VMware Workstation, such as incremental snapshots, USB 2.0 support, raw partition support, and developer tool integration in the public beta of version 6. You can participate in the current beta by visiting http://www.vmware.com/ under the current beta programs link on the main page. Keep in mind that this is not supported directly, and has debugging information built-in, so it might be slower than their release products. On my Core 2 Duo T7400 system and my Core Duo T2600 notebook the beta name and debugging slowdowns don't affect most activity. Give it a try, trust me it'll work with Solaris, I use Solaris 10 U2-U3, and OpenSolaris Express B50/B54 with it, and Parallels Desktop (B1970) and they are fast and work without errors with Solaris.
Sorry if you intend to use VirtualPC, I know Microsoft is giving it away for free, and I do have experience with it, and their counterpart Virtual Server product, but their BIOS implementation is immature and incompatible with legacy and non-Microsoft operating systems for a variety of reasons. They won't support it, neither will Sun or the community, so my recommendation is to consider one of the other emulation products.
With regards to the earlier post Virtual PC being a Microsoft product, that is about the end of the factual information in that post. I have Virtual PC 2007 installed and am currently running Fedora Core 6, SuSE Linux Enterprise Desktop 10, in addition to Windows XP and Windows 2003 Server.
Now, if someone does in fact figure out how to get Solaris to boot properly that would be great, I don't recall having this issue with Solaris 9.
Check this posting I made on Microsoft's site:
http://www.microsoft.com/windowsxp/expertzone/newsgroups/reader.mspx?query=sola ris&dg=microsoft.public.virtualpc&cat=en-us-ms-winxp&lang=en&cr= US&pt=&catlist=B0DE109D-10E1-4C3C-BCC9-8EB7A22FC6A0&dglist=&ptli st=&exp=&sloc=en-us