Problem opening big sessions.

[JD85]

Pro Tools LE 7 with *Mbox (1) Connected through USB

Computer:
Windows XP
Compaq CQ5110y PC
AMD Athlon 7550 Dual Core Processor
2.51 GHz, 2.87 GB of Ram (System Memory 3GB
320GB Hard Drive

Added Plug Ins:
Waves Diamond
Antares Autotune

This computer came with windows 7 but the mbox (1) didn't want to work with Windows 7 so I installed XP on it. The recording sessions are perfect and the mix sessions too. The only problem is that after I add all the effects and mix the track down well, I cannot quit the session and reopen it. Windows shows the "END NOW" messeage and then takes me to the "SEND REPORT" window that comes up briefly after. I don't get an error number at all. Is this a processor (RAM) problem or is it the mbox (1)? I need to know what improvements to make (buy) on my system for this not to happen as it totally constraints my ability to open a session the next day; I have to finish sessions on the same day because they wont open later.

Thanks.
*That is the first mbox