Generally there's no need to optimize Windows 7 Virtual Machines. Unlike previous Windows releases it's pretty smooth right out of the box as long as it's got at least 1Gb RAM allocated. I'm using Windows 7 x64 under VMWare Fusion on OSX and it's blisteringly fast.
We run a lot of Windows VMs for software testing on a dedicated ESXi machine (about three each of 2000/XP/Vista/2003/2008/2008R2/Vistax64), so I can provide a few tips that apply Windows VMs in General.
- Turn off all desktop effects, use "Windows Classic" mode
- Disable the SuperFetch service (heavy on disk I/O and memory)
- Allocate as much RAM as you can to the VM
- Turn off Windows Defender
- Install a lightweight AntiVirus (at least on 2000/XP/2003)
- Use "High performance" power saving mode (i.e. none)
- Disable Screensaver
- If possible, put the virtual disk image on a separate physical drive. But make sure it's not on a software RAID array.
- Go get some more RAM and allocate that to the VM as well.
My experience is only with the VMWare family of virtualization products (Workstation, Server, ESXi and Fusion) and I haven't come across any issues with Windows 7. I'd suggest downloading the VMWare Workstation and seeing if there might be a bottleneck somewhere with the VirtualBox implimentation.
Remember, system requirements for Win7 states a minimum of 1Gb RAM. Minimum. Performance is good with 1Gb, but give it less and you're asking for trouble.