This is what I suggest for you to fix, remove, or repair on your system(s).

1) Turn Animated Icons off and use a standard mouse cursor. Extra memory and graphics RAM being used on something not needed – Sounds petty but it does improve performance.
2) Was an Anti-Virus running when SW was installed? If it was, a reinstall would probably need to be done. AV can cause some files to not be installed and SW performance and stability can be compromised.
3) If you have numerous processes running at an excess of 40-60. Reduce that number if possible. Some sites to help you with processes running http://www.blackviper.com/WinXP/servicecfg.htm- XP http://www.blackviper.com/WinVista/servicecfg.htm - Vista
4) Remove and uninstall all applications that are not needed (Shareware programs) like the Weather bug, etc… - These Applications use up needed RAM for other more important apps. Some Shareware programs have Hijackers in them and can cause further slow downs of your network and Internet activity.
5) Check for Hijackers and spy bots. These items can cause memory loss and unneeded network traffic. Check out software to help with Hijackers these programs help you to get help to find hijackers on your system. There are many applications to help with Spyware, but this is one I recently found that helps me out tremendously. Super Anti-spyware get the free version, or if you want to pay for it that is your choice.
6) Cut out the complexity of your models. - e.g.: If you are not a chain maker, don’t make all the links and place them in your models. Make a solid band to represent a chain.
7) If there is a rebuild light in your part model or of an assembly. This will slow the system down, because it tries to rebuild file in the process of it opening.
8) Any errors in a model will cause slow downs in your opening process
9) If you are exceeding the amount of RAM you have, you will start paging the HDD it will become a lot slower in opening the file and working with it from there. Get more RAM or 64-bit
10) Clean your TEMP folders out regularly. http://www.scottjbaugh.com/Tips_Tricks/Computer_Maintenance.htm
11) Is you Minimum and Maximum Virtual Memory set to the same value? It should be, so too not allow the file to grow in size.
12) http://www.solidworks.com/swexpress/mar04/200403_techtip_02.cfm - Check here for “Overall System Performance”
13) Is your video card and driver certified? - http://www.solidworks.com/pages/services/VideoCardTesting.html?pid=252
14) Is it an older video card that is supported? If its old, it may not really support large assemblies. There are certain graphics cards for these types of workloads that you do. – e.g.: The Quadro 350-700 FX is a low-end card for SW. Works well with about 50-400 Component assemblies. Quadro 1300-1700 FX is a Mid-Range video card. It works well with more complex Assemblies maybe 100-1000+ components. Quadro 3000-5600 FX are the high-end cards and should support larger assemblies of 1000+. SW supports all of these cards, but each out performs the other lower one.
15) If you are opening the files over a network, you should really try and open them locally, but if that is an issue for you. Then there have been some discussions of Network problems and possible fixes on the web.
16) PDMworks will also help you and should be highly considered if you continue to work the way that you do now.
17) Maybe enabling the 3GB switch in XP might help you out - you musth ave at least 4GB 32-bit XP machine. http://www.kcswug.com/documents/3gb_switch_part_one,_two,_&_three.pdf - This is a use at your own risk. - I have tested this and it works well with 4GB. 3GB caused instability.
18) What is your CPU size or speed? If it isn’t up to date, you might consider moving up to a faster CPU of today or go Dual/Quad Core
19) If you have a system that has Hyper-threading enabled. Disable it in the Boot up process. This allow SW to use the entire CPU, instead of only half of it.
20) Increase your RAM to what the Motherboard will support – I don’t recommend anything under 2 GB for SW08.
21) What is the number of your Top-level mates? It should not exceed 300. If it does then you are going to take a performance hit. To lessen the number of top-level mates, make more sub-assemblies. For every one part you must have at least 3 mates to define a part. If you 125 parts in an assembly, that equals to 375 mates. If you make those 125 parts into 6 sub-assemblies, that equals 18 mates in total. That lets SW rebuild only 18 mates versus 375 when opening. This will increase performance and open times.
22) If you are having network related issues while working over a network. You should consider getting the Engineering group off the main Network and have them work on their own network, but still have access to the rest of the company.
23) Test run an assembly on your local drive to see if your performance increases and the amount of memory being used is not exceeding the amount you have? This is recommended to ensure the issue is network related.

I’m sure some the options above will help in keeping your Assemblies running smoother.