I installed Ubuntu 11.04 from Vista. It was installed into another partition which I made with Acronis Disk Director. When the computer booted up, a boot menu was displayed. I now think this boot menu was wubi. I guess the MBR boots into Vista and wubi provides the boot menu. However the Acronis OS Selector does not recognize Ubuntu. I now cannot boot into Ubuntu, which a put a lot of stuff into, because Acronis apparently did something to the MBR.

It looks as if a real Linux installation takes three partitions and the boot is probably off one of those, possibly root?

If this is true, how do I set up the hard drive so that it will boot off the Acronis OS Selector, which looks like a standard boot utility. What other boot utilities are there? How do I boot into my old 11.04 system? It looks as if Wubi only gives 256mb of something (disk?) to memory cache, whereas the recommendation seems that memory cache should be at least as big as main memory.

I think your first mistake is installing Ubuntu while inside Windows. This method creates a virtual hard drive image inside a Windows partition and uses the Wubi program to initiate the boot from within Windows. So you're not really dual booting if you are using Wubi.

The boot manager obviously would not be able to boot that partition because it's just a Windows partition with a large file (the virtual hd) inside.

If you really wanna dual boot, there really is no other option besides backing up & reinstalling it again the right way. By booting from the Ubuntu CD.

A wubi install of Ubuntu puts Ubuntu inside windows as a program and it is not on a separate partition.
I've never used wubi but I think you can use its' bootloader (Grub) to boot it and windows.
I'm not sure what the default for an actual install of Ubuntu 11.04 is. You generally only need one partition, usually also recommended to create a second partition for swap.

Did you actually do a wubi install inside windows?
Did you have the Grub boot menu previously when booting?

Use your Ubuntu CD, boot from it and go to the Applications tab and find the terminal, open it and run this command:

This should output your partition information which you should post here to get help.
I'm not familiar with Acronis so I don't know what might have happened there but I'm sure someone else will be able to help.